I often use US Post Office Flat Rate boxes to ship extra stuff home. I buy the postage before traveling and pack the boxes in that flat form in the bottom of my suitcase.

I will do it again soon when I have to start a trip with a few days in a cold climate and fly on to a warmer place. The severe weather clothes will go in the Flat Rate boxes and my travels will be easier.

Once filled and given their postage, you can drop a Flat Rate box off at any post office in the US using the bin inside the post office.