I just did some testing, and sure enough, it works! Thanks for the tip!
This method will take a while to set up, but should work perfectly for what I have in mind.
I don't mind taking the time to do it, but I have a few more questions first.
Is there a limit on the number of shipping rules that I can have?
I think I may need a lot of them.
Up to 13 for First Class Domestic, then about 25 to 100 for heaver ranges.
I haven't done the math, or rate comparisons between different domestic couriers,
beyond 15 pounds, just yet.
How many digits can I define past (to the right of) the decimal place, in the
"Shipping Amount Per Ounce" field?
Does "Price Per Unit Weight" ignore the Box Type selected when setting up an item?