I'm not sure about the envelops (flat rate), but I know that the boxes, you can send up to 20 pounds for the flat rate internationally at one price. I'm still trying to get 20 pounds into one. Have gotten up to I think 13 pounds at the most. lol
Customs forms can be a pain in the arse, but yes, make sure you read the fine print about them. The forms have changed too. It used to be this nice form where you could write in big letters, now it's smaller blocks (I don't like that).
And yes....I have to fill them out from here (Guam) going to the US even tho Guam is a territory of the US. Go figure.