I can see where the OP is coming from.
If I made an order for a product over the phone and was going to pay by wire (which, by the way, is perfectly acceptable as long as the company advertises it as one of the ways they choose to do business), and I was given a total of $2148, I would then double check: "Okay, I'm going to be sending $2148, right? That's the grand total?" Then I would do my part by getting the wire arranged and sending the total.
If, the next day, the store called me back and said "Oh, sorry, we were wrong, your total is actually $2548, we made a mistake," yeah, I would be annoyed. Whether it was tax, or they priced the product wrong, or they forgot to add the shipping..... doesn't matter. I was given a total, paid it, and now they are asking me for more money. That's annoying. Even more so when I have to make special arrangements again.
It's not like the OP was specifically trying to get out of paying sales tax.
So yes, Apple made a mistake. It was an annoying one and I would have been cheesed off too. I probably also would have asked them to honor their price (they would still have paid the sales tax but obviously would have had to take it out of the total they had already given me, as that was supposed to have included the sales tax).
What they did was more than they had to, as they could have told the OP to go jump, and either pay it all or forget buying an Apple computer. Of course it was less than saying "Okay, our mistake, we'll eat the extra $."
It's nice they didn't go the total "jerk" route, and I think what they did was decent. I always appreciate it when a company apologizes and at least tries. But I can also see why the OP was a bit miffed, and I don't see where the OP was trying to cheat anyone.
MT