I would check with the first class stamp thing
they don't just go on size anymore, they include weight. If you can weight it and measure it at home you can price it up online.
( Mrs Sp@m, Tue 16 Jun 2009, 9:34, archived)

well it's quite light - at least lighter than most bills
I'll stick two on to be safe though.
( SuperMatt, Tue 16 Jun 2009, 9:35, archived)