Even with the fastest shipping you wouldn't get a Sunday release until Monday, maybe Tuesday (depending on whether they ship it o Saturday or Monday).
To me, this is a no-brainer. It's not like you NEED to play the game on launch day, so take the $20 credit and wait a few days to get it.
Mop, sales tax depends on where you live. Here in New York, for example, all online purchases include sales tax (this was passed last year, but Amazon continues fighting it and are trying to get it repealed). Not a huge deal, but for a $50 game that is a extra $4 that could be spent on something else.