I've never registered a domain with GoDaddy before, and so I had "problems" and I'll post this in case anyone else has problems.
I couldn't find the place where you enter the coupon code, so I called customer service. (Annoying, pre-recorded twat wouldn't STFU, so I was in a rage when the Customer Service guy took my call. Also "press 1 for English" is also annoying, /rant.)
Bought my domain for @ $2.57 (thanks OP), and then learned that the coupon code entry-point is in the screen just before the screen where you enter your credit card information, somewhere in the bottom-right, I think he said.
I didn't know if it was after you click past the credit card information screen, and didn't want to get billed full-price to find out that it wasn't. I'm posting this so that someone else doesn't make the mistake I almost made.
Coupon code is good. Someone should Rep+ the OP.