So I'm pretty sure I've cheated and used magic10 at E more than once with a signed in acct. I could be mistaken, but I think I've used it twice. here's how:
If you add items to your cart while logged in and forget about them, after a few days, maybe a week, E will email you reminding you that you forgot to checkout, and urging you to save 10% when you do with the code MAGIC10. I think this email re-enables you to use it on your acct. Again, I could be hallucinating but I swear I've used it more than once.