The only way it would cost that much to post something of that size is if the mailer were to post by registered mail. Basic cost $7.90 for the registry fee + applicable postage (probably 84 cents, as advised by Nanci). This may or may not be what you want--registered mail is handled by hand only (versus being processed on automated equipment), so the possibility of your decal getting bent is minimal--but otherwise it sounds as if he is trying to charge you excessively for 'handling'.
Also, although USPS does have a 'tiered' system for various global destinations, England (U.K.) is not one of the higher rates. In fact, for something like a decal he probably doesn't even need to go to the trouble of making up a customs declaration (it could be considered 'written matter', and not dutiable).
I would ask him what type of shipping label he is creating that does not allow him the option of airmail letter rate...
East Hill