Well, since it appears that I can only test this problem on a live site (since it occurs in Step 3 of 3 of checkout and needs to go to credit card processing), I'm not going to be able to do this before the holidays. I can't afford any site down time right now. I don't have money orders installed, but I can certainly give that a shot...
However, I did put together one more piece of the puzzle. I looked up the 2 gift certificates that went through successfully (one on 12/6 and another on 12/10) and realized that both customers bought something in addition to the gift certificate. I tested this, and sure enough, I don't get the error if I'm purchasing a gift certificate along with another, non-gift certificate product.
I'm wondering if you can point me in the direction of some specific files that I might look at that might be causing the problem? I have a recent, full back-up of the site and I'll try using Beyond Compare to look at that versus the clean, 1.3.6 full file set. I've never used Beyond Compare before, so it might take me a while to figure out what I'm doing!



