When I moved internationally, I found out about the ‘M-Bag’ service. The post office gives you real mail sacks (hefty, expensive seeming things!), which you can directly fill with books and printer matter (and nothing else!). They’re then tagged after sealing the drawstring, and shipped internationally!

I’m sure the USPS wants those sacks back, but the post office in the UK, where I had them sent, was just perplexed by them and told me to keep them.

https://faq.usps.com/s/article/What-is-M-bag-Service