Do you mean you want to use a webbie in a Build a Bear store, or are you planning to use it at the online Build a Bear?
If you plan to use it in a store I'm not sure if they'll accept a printout from Webcertificate with the number on it. If you get a card you'll have to pay a fee to get one.
I've used webcertificates online with no problem. I've never tried to use them in a physical store.