I am not quite sure how this works, but if you partner with Amazon they assist with the shipping. I know booksellers that send their books to Amazon and Amazon takes care of the shipping. Best of luck to you!
I too run into issues with shipping in my business. The one tip that works for me (hit or miss) is Courier Buddy. They have helped me with my online store quite a few times over the last year. They're like the Uber of the courier industry. Sorry I don't have more insight for you...just wanted to share this one solution that works for me from time to time.
I recall the old "SCORE" that assisted small business people. The "Service Corps of Retired Executives" I think it is. They say "For over 50 years, SCORE has served as America’s premier source of ____free____ [my emphasis] business mentoring and education." Maybe they will have someone who can help you. See https://www.score.org/