Every contractor for large corps that I know has at least one story about chasing up payments and researching small claim court rules. I'm not exaggerating - every single one I talked to. It's sometimes "NET please forget this, and we'll try to drag it out anyways.".

Also the terms should be an agreement, but in practice it's: the large company has more power and will tell you their finance dept will not pay out before X - doesn't matter what you put on the invoice.