Depending on your digital goods, have you thought about sending a link in email, as part of a receipt even? Could easily setup a mail server to forge the time back 15 minutes and make it look like it was instant and just not send it if it was fraudulent. It's a little sleazy but it'd at least give a completely different impression than, "Please wait 15 minutes".
People just don't want to wait that time at all. The actual impact on user's hasn't been too bad (because I'm taking the risk), but the actual flow, from my perspective, has been pretty poor, and from what I can tell it's not documented that they'll hold transactions for 15 minutes.
