Yeah, you would have to contact any designer who's stuff you were planning to use, to see if they even allow for that use. Most don't, so you should definitely check. Some will, with an additional fee, but again, that's up to the individual designers.
ETA that after being in the greeting card business for years, you also have to make sure any fonts you use are commercial use as well, always read the TOUs that come with them when you buy them, you'd be surprised by some!
