Recently, we (USDA, SEL at The Smithsonian NMNH) were informed that USPS came up with a new customs form which must be completed on line at not cost. Once you complete the form, it will generate a barcode along with all the information required (sender/recepient info., weight, value, etc..)
I have to say that in the field about value ($), we were informed by the shipping office from SI to put $1.00.
I made some sort of instructional (step-by-step) on how to complete such form.
I hope this helps... Recently I sent two packages to UK(NHM) and I got a confirmation email that they made it there with no problem.
Last but not least, do not forget your eDec (https://edecs.fws.gov)
I must remind everybody: this ONLY applies to packages sent FROM US to the rest of the world.
USDA, ARS Systematic Entomology Laboratory
Smithsonian National Museum of Natural History