As far as I know, you simply put down the full amount instead of a partial amount. I'm not 100% sure as I didn't end up purchasing through Gamestop; I asked them about things because I was planning to, and when I was informed that it was just a money code for the exact amount (which doesn't include whatever tax it'll cost [which I really should update the post to include, as that can and will cause people issues, now that I think about it]) I ended up just buying money cards until I could pay for it and the tax.