Hi Robin, and thank you for posting to our community!

Typically USPS requires more postage because an item wasn't weighed properly or sent via the correct service option.

We have some general guidelines listed in the link below to help you get your shipments packed and shipped out properly, but you will always want to make sure you are weighing your items after boxing for the most accurate weight.

Know How: Protect your shipments + ensure delivery

We also have some additional information regarding USPS international services and their restrictions listed in the next link.

What options do I have for international shipping?

Thank you.