For several years, Robin Cooper has been plaguing department stores, hotels, associations, fan clubs and a certain children's book publisher with his letters. From Prince Charles to the Peanut Council, Harrods to the British Halibut Association — no one is safe. So who is Robin Cooper? Architect, thimble designer, trampoline tester and wasp expert, Robin Cooper is all of these things — it just depends on the person he's writing to…