I found this on https://ircalc.usps.com/

First-Class Mail® International Letter**
Value of contents can not exceed $400.00
Max. length 11-1/2", height 6-1/8" or thickness 1/4"

$1.15


So, pretty cheap! I sent a tiny amigurumi Squirtle I made to a friend in London and it cost like $25 because it wasn't a flat envelope @_@