If you're shipping documents via Priority Express International you can use a USPS Flat Rate Envelope or you can ship by weight if your package is over 4lbs.
Your questions about Priority Express International answered:
How long does Priority Express International take?
Priority Express International delivery times will vary depending on the country you’re shipping to, but most major destinations in Europe, North America, Africa and Asia can be reached in 3-5 business days.
Will my parcel be tracked?
Yes, your package will be fully tracked to its destination.
What's the weight and size limit for Priority Mail Express International?
Maximum weight for mailing boxes is 70 lbs
Maximum weight for Flat Rate Envelopes is 4 lbs
Maximum combined length and girth (girth is the distance around the thickest part) is 108″.
Where can I drop off my package?
You can drop off your package at any USPS post office or drop off point.