I don't believe there is a minimum purchase. I don't recall anything in there being less than $15, so that shouldn't be a problem. I used it twice on $20 games before = $10 shipped.
No code for me yet, but I'll post it when I get it since I've gotten the only EA games I need right now.
And these are all one-time use codes, so if the ones posted don't work, it's because someone used it and didn't say so.