I tend to work on fairly consistent small sizes, so when I started on here I picked up a pastel-picture, along with its intended protective boards, paper and packaging envelope and weighed the lot. Having got the weight, I then researched the costs to post such a package within my own country (England) and overseas. I used our Royal Mail charts on the Internet for this. Did the same thing for my few small oil paintings as well.
Having got the costs, I then had to convert them from sterling to dollars (you may not have to do this, dependent on where you live).
So far, everything I’ve sold has gone to the USA and all have arrived in one piece. Posting from England I have to attach a customs label as well.
So, if you get the weight and prices for one of your typical-sized works plus packaging, it should act as a broad guide. You can then input your postal prices more confidently when you upload your work.