Selling bike parts for me goes in fits and starts. I will go for months without even listing anything, then — as I did twice in the past week — list a number of items. This last time, I had a bunch of freewheels to get rid of. They sold in groups of one to five. Even a single freewheel was too heavy to go first class. Two would fit in a small flat rate priority box, but three needed something bigger. Medium flat rate was overkill — box way too large & price way too high — but it turns out the Regional Rate Box A was just right. It does have a 15lb weight limit, but that should be fine for most stuff.
I wound up spraying freewheels all across the country. None of the boxes weighed more than about five pounds. One went to NJ for $8.92. Another went to OR for $11.13. One went to OH for $7.92, so even cheaper than small flat rate. To top it all off, it sorta fit in my handlebar bag:
The only catch I found was that you can't get these boxes at the post office. You have to order them. So I ordered a bunch against future needs. They have both top loading and side loading versions. I'm hopeful the side loader will hold a crank. There is also a "B" box which is a bit bigger and has a higher weight limit. Full details here:
https://faq.usps.com/s/article/Regional-Rate-Boxes