I have no idea if this is possible or not...but an idea...what about using the "download" option? You could have a table with games listed and the price you would pay...you could then have the customer "download" a RMA...sort of thing...custom programmed to include a special code...order id? that when the customer mails you the game....you match up the rma (order id)...and then you would know what you were to pay them....
If you need to keep track of what games you have agreed to pre purchase...like an update on the site...will buy 6....and then when 6 have been "downloaded"....you will accept no more...not sure if that is possible...or is what you need.