Update
Loading the 1.3.9g versions of the "changed in 1.3.9h" files has it working again. I actually just loaded the non-admin files for now.
One interesting thing I noticed before reloading the old files - when I got the error, if I clicked on the product in the shopping cart and went to that page, then clicked on checkout, it would let me do it without throwing the error a 2nd time.
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