I asked this exact question today when i put down some more money on mine. Turns out that you are only charged sales tax on the cash you put down. So if you pay all with credit, then no, you will not be charged sales at all. but if you do lets say 150 cash and 200 credit, you will be charged sales tax on the 150 cash.
Is that tied to the state by state tax thing or is it universal? I live in a state that taxes on GS credit and have never paid off any pre-order. Reason being, I never pre-ordered anything at GS before this WiiU. Not even to get a bonus and then cancel.
Can anyone who lives in a state that gets taxed on items paid for with GS credit confirm either way? If true, I will find flips or whatever to put towards the WiiU to save $21 tax.