USPS priority mail – which options to select?

I'm shipping books, which in small quantities fit into envelopes or small boxes, but in large quantities may require a carton.

Media Mail shipping calculator works fine: there is only one option ("media mail") and that copes with larger quantities.

But with Priority shipping there are many options to specify. I tried just "priority mail" but that often produced the wrong result.
I found that "priority mail medium flat rate box" worked most of the time.
But when I needed to ship something bigger than a medium flat rate box, the result was completely wrong – about $43 too little.

Can you advise which priority shipping option would be the best one for me to choose? As I understand it, you don't use the dimensions of the products so there's no way for the calculator to decide what size box to use.

Thank you
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