[quote name='KoRuPT']Yes, they do stack, up to $500 I believe.[/QUOTE]
Now, I am not sure, but from what I think I have read, it depends on if your Paypal account is verified or not. I read on SD I think that unverified Paypal accounts can only stack $500 worth of gift certificates/promo codes, while verified accounts can do $2000. But I may be remembering something wrong and be confusing it with something else I read.