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Dispute codes can be confusing, we provide this 'cheat sheet' document here to assist in understanding what the code might mean and how ling you have to rebut a disputed charge by clicking this link: dispute_reason-code-cheat-sheet.pdf
BookingCenter customers present the the required 'Must Agree to Deposit and Cancellation Terms' checkbox in order to complete an online booking or self check-in, as do major OTAs (Expedia, Booking.com, etc). With the action of actually 'checking' the check box, this covers you in the event of a dispute about your policy.
However, no one can guarantee a user actually reads it :-).
Yin the event of a chargeback dispute, you will show that a box with your clear deposit and cancellation policy was clearly presented- and it's required to be physically clicked - to assure the merchant processor that there was no way the Guest could have booked or self checked-in without physically clicking the box where they 'agreed' to the terms. Usually a screen shot with the log events (found in the actual booking under the 'Log' tab) showing when the Guest did the events - booked online and/or self checked in - will cover the documentation required to show that guest did 'agree to the terms'.
You can never show that an email was actually read or received by a Guest, as the internet does not allow for anyone to assure emails reach their intended recipient. However, we recommend that our properties setup a 'CC' email that goes to a free web email service such as Gmail or Yahoo and CC all Confirmation and Cancellation email Letters to that address as verification that it was sent on a particular date and time, and their booking email was the 'To': recipient (learn how to set this up here). Presenting that as evidence is the best you can do to clear up confusion - the email was sent, as evident by your CC copy.