Sorry to sound dense, but do you mean postcard-sized card (i.e. blank both
sides) or marked on the back? I've been producing postcards for years, using
MS Publisher, by quartering an A4 sheet (210g/m² glossy photo paper)
manually adding the markings on the back, and then trimming to size. If you
have Publisher I can send you a template. If not, I can send you a PDF of
what it looks like.