BookingCenter has a good Chargeback Letter that can usually win a disputed credit card charge assuming you provided the 'product' that the guest booked. The letter can be made available to your MyPMS and is called "CHBK: Chargeback letter." if you don't see it in your Letters setup area, request it from our Support Department, and we can add it for you.
Dispute codes can be confusing, we provide this 'cheat sheet' document here to assist in understanding what the code might mean and how long you have to rebut a disputed charge by clicking this link: dispute_reason-code-cheat-sheet.pdf
BookingCenter customers present 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 the 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 set up 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.