We had to replace an A/C unit in the new house just last year. Old one crapped out, main compressor locked up. Turns out it was about 14 years old and undersized for the house. Our supplier showed up with FPL rebate certificates and stuff when they brought in the new one. They gave us a very specific receipt for use in declaring federal tax credits.
I'd say if your supplier doesn't hand you that sort of thing, find a better supplier.