In a shockingly short amount of time, I had a response to my Powell's nastygram. At first, they offered to just give us free shipping on our next book order. I was prepared to be satisfied - it would be strictly fair of them to give us that. But then Powells.com's customer service manager, a woman named Beth, took the time to write a letter explaining the situation - why things happened the way they did. She improved their offer to not only free shipping on a future book order, but a free copy of a guitar book similar to the one we wanted and enough credit to bump the $22 gift certificate to a no-longer-awkward $30.
This is the Powell's I remember. And I think it's safe to say that we can all be friends again. Thank you, Beth!