Any ettiquette book will tell you that it is not necessary to send a wedding gift when one has declined the invitation. Miss Manners refers to idea that invitations require gifts as a “shakedown.” Also, it is not necessary to explain why one has declined the invitation. All politeness requires is that one response to the invitation. Either send back the reply card or if there isn’t one, just send a note declining. The traditional wording to decline a formal invitation would go like this:
Code:
Mr. and Mrs. John Smith
regret that they are unable to accept
Ms. Jones and Ms. Jackson's
kind invitation for
Friday, the fourteenth of June
You get the idea. If you use the traditional wording for declining an invition, you will be perfectly polite, and you should not offend anyone. Your reasons for declining are no one else’s business.