Priority Mail Box - 4 | USPS.com
This unique, cube-shaped box accommodates a variety of items such as clothing, books, and hats.
store.usps.com
I am finding this to be the most cost-effective option for mailing 2-3 samples at a time.
Packaging is free. USPS Priority Mail, at least for my circumstances, was $8 from CA to UT for Polaris.