I once read somewhere - and I can't remember which book or author, unfortunately - that including your business card with a thank you note you send someone is not a wise thing to do. I always thought this was a little far-fetched. Why not put that little piece of marketing in there? What's the harm, really?
Over the past few months I've received several thank you notes from other wedding professionals, some with business cards included and some without. Turns out that author was right: the notes that included the business cards felt less valuable and disingenuous. They didn't carry the same weight because they came with the unspoken message of "I am only sending you this thank you note so I can put my business card and company information in front of you one last time". Unfortunately, whether that was the sender's intent or not doesn't really matter; it's still the way it came across.
Next time you send a thank you card or just a hand-written note in general, omit your business card. Say thank you simply to say thank you. No "what's in it for me" or strings attached.