First, when testing whether you can change a price and have a transaction go through successfully, RAISE THE PRICE. If you lower the price the affected entity may come back and say "See??? He's STEALING from us! Lock him up!" If you've overpaid for something through their web interface that complaint and issue goes completely away.
Second, if you're going to suggest that they contact you for assistance in fixing it also suggest other options. My typical handling for this is with hacked websites, so I'll basically say "Your website has problems X, Y and Z. You should work with whoever you have working on your site to resolve these. If you don't have anyone I may be able to assist you, or I recommend talking with a firm like Sucuri.net which has dealing with and preventing issues like this as their primary business. (My only link with Sucuri is having seen some of their folks do presentations at trade shows.)"