Consideration does not have to be equal, so get creative.Just remember that whatever you agree to is binding on you as well, so follow through.
So if your cousin offers to do your wedding photos for free, and you don’t have to promise anything in return, that isn’t actually an enforceable contract.
That’s a gift, and you can’t (usually) go to court to enforce a gift. Try to figure out a way to provide consideration of some kind in exchange for whatever they are offering.
You don’t want to find out that you have no wedding cake because the baker was waiting for you to pay a deposit. If you don’t feel comfortable paying sixty percent up front, say so.
And for goodness’ sake, don’t pay a hundred percent up front unless you really know and trust the vendor (as in, it’s your mom).
An offer, acceptance, and consideration on the part of both parties.