An 8x10 bubble envelope is going to count as "small parcel" and any small parcels up to 100g to the US cost £3.50
Each 1st Class stamp is worth £0.60
In the UK generally celebs don't send 8x10s but 6x4s so you'd be fine with perhaps a 7x5 envelope (assuming you are not sending something to be signed).
A bubble mailer however will count as small parcel irrespective of size.
It's also a good idea to write "SMALL PACKET" on the top left hand corner of your package.
