An e-ticket can be returned with the appropriate charge and within the time limit before the departure of the connection through the IDOS.cz website on the Tickets page (https://idos.cz/jizdenky/) by filling the unique IDOS.cz Code (its format is XXXX-XXXX-XXXX) of the e-ticket into the appropriate form. This code is always present in the text-body of the delivering e-mail with the e-ticket and in the name of the PDF file of the e-ticket itself as well. The Code IDOS.cz is also part of text content of the PDF file of the ticket (this does not apply to tickets for connections of the Carrier České dráhy).  
When returning the ticket on the same computer and in the same internet browser where it was purchased, you can return it by clicking the "return ticket" button, if the e-ticket purchased by you is still visible on the listed page.   
An e-ticket purchased by a certain logged-in user can only be returned by the same logged-in user, and it can be easily done via mentioned button without necessity to enter the code of the e-ticket into the form.   
You can also return an e-ticket of the Carrier České dráhy directly in the e-shop of the Carrier České dráhy (https://www.cd.cz/eshop/vraceni-a-vymena/) using the six-digit code of the e-ticket, listed on the ticket (transaction code).

The return conditions are determined by the relevant Carrier, the e-shop of IDOS.cz has no influence on them. Always before the payment process is started, the return conditions relevant to the e-ticket are included in the “Conditions of carriage” link under the name of tariff of the e-ticket in the shopping cart.
After purchase, return conditions are visible in the PDF file of the e-ticket, usually in the format “charge in €/time before departure in hours”, means - the time limit until which the Carrier allows to return the e-ticket with the application of the specific charge.  An exception is the Carrier České dráhy, the return conditions are listed on the Carrier's website - domestic tickets can be returned no later than 15 minutes before the beginning of their validity with no charge to be applied and international tickets are refundable until 00:00 before the first day of validity with a maximum charge of €3 per person. Some of the e-tickets of the Carrier České dráhy are non-refundable (this information is presented on them).
OneTicket e-tickets can be returned no later than 15 minutes before the departure of the first train from the connection.
 

Tickets cannot be returned usual way after the scheduled departure time. In such a case, they can only be claimed. Information on how to rise a claim are stated HERE.  
Only the Carrier České dráhy allows a domestic ticket to be used to travel on some of its later connections, but only if for the ticket is not defined so called “Obligatory train” to be used. The journey must begin on the first day of the validity of the ticket.

After the scheduled departure time tickets (even doh they are unused) are non-refundable.  It is possible to rise a claim in certain cases, information how to rise a claim are listed HERE.  
Only Carrier České dráhy allows to use its domestic train tickets for a next train(s) operated by ČD during the validity of the ticket; applies only in the case if the ticket not specified an “Obligatory train” for the relevant route. The journey on the alternative (next) train has to begin during the first day of the validity of the ticket.

 A multi-passenger group ticket can be returned in its entirety only, it is not possible to remove a passenger separately.

Tickets cannot be returned after the scheduled departure time, so if you miss the connection, the ticket is forfeited. 
Only Carrier České dráhy allows to use its domestic train tickets for a next train(s) operated by ČD during the validity of the ticket; applies only in the case if the ticket not specified an “Obligatory train” for the relevant route. The journey on the alternative (next) train has to begin during the first day of the validity of the ticket.

Only the Carrier České dráhy (ČD) allows to return the ticket even after the expiration of the time-limit for returning the ticket (for ČD domestic tickets the time-limit is no later than 15 minutes before the beginning of their validity). The condition to return the redundant ticket is to prove that you bought also another ticket for the same journey, then the carrier should refund the fare for the unused redundant ticket.  
You have to fill in the six-character alphanumeric ticket code on the České dráhy e-shop page (https://www.cd.cz/eshop/vraceni-a-vymena/) of the ticket that has not been used and the e-mail address used during the purchase. On the return form select the reason for the return of the ticket - set “other” and specify that you have mistakenly purchased a ticket twice and fill in the text explanation of the form the six-character alphanumeric ticket code for the ticket which has been used. And ask for a refund for the returned unused redundant ticket.

Conditions applied for the return of tickets strictly follow respective rules defined by each Carrier.
Separately issued tickets (one for the way There and another one for traveling Back) of the Carrier České dráhy is possible to return individually for each of the ways of travel. If the ticket of the Carrier České dráhy is a type of ticket, so called “network ticket” (for example an “all-day-ticket”), then this specific ticket is returnable only as a one whole indivisible ticket, and before its validity begins.
The two-way universal train ticket OneTicket can only be returned in its entirety at IDOS.cz, i.e. the trip There and Back together, and before the start of the validity of the relevant ticket.
Two-way tickets (There and Back) with a special discounted two-way tariff, issued with one check-in code for both directions, which are offered for some bus lines, is possible to return just one way or both ways together. The return of the ticket is allowed only until specific defined time before the departure of the relevant (to be returned) way of the ticket. If for two-way ticket is returned just one of the ways and the departure time of the other (not returned) way is still in the future, then our e-shop will generate and will send to you a new one-way ticket (for the way, which has not been returned) with a new check-in code, however the special discount applied previously for the initial two-way ticket will be not applied for the new one-way ticket (the way of travel of the initial two-way ticket, which was not returned). In case of partial return of two-way ticket the refunded amount for the returned way is equal to the discounted part of the two-way ticket; therefore the refunded amount is lower than the price for a regular one-way ticket on the same bus line.

It means that the code of the ticket has been filled in incorrectly. When returning a ticket, there has to be filled the IDOS.cz Code of the ticket, its format is XXXX-XXXX-XXXX. The IDOS.cz Code is listed in the information text of the delivering e-mail with the ticket and is also presented in the PDF file of the ticket itself (this is not appliable to tickets for the connections of the Carrier České dráhy).
Beware of typos caused by mistake substitution of characters – e.g.:  letter “O” and number “0”; letter “I” and number “1”; letter “S” and number “5”; letter “B” and number “8”, etc.

Only the Carrier České dráhy allows to return the ticket even after its deadline to be returned (for ČD domestic tickets it is no later than 15 minutes before the beginning of its validity), however you have to prove that not all the passengers from the group ticket used that ticket, then the Carrier should refund the corresponding part of the fare. Therefore, you must request a confirmation from the train staff (the conductor) about not used part of the ticket. 
After you have the confirmation, you have to fill in on the České dráhy e-shop page (https://www.cd.cz/eshop/vraceni-a-vymena/) the six-character alphanumeric ticket code and the e-mail address used for the specific purchase. Then you have to select the reason for the return of the ticket - set “other”. Specify that the ticket was not used by all persons and attach the confirmation issued by the train staff about the incomplete usage of the ticket. And ask for a refund for the relevant unused part of the ticket.

Returning of a ticket is an irreversible process. The returned ticket is no longer valid and cannot be restored in any way.