OK, I understand the confusion and apparent lack of logic. not saying you, Dave. but, folks have abused the free Priority supplies since the beginning. they used to be plain inside and some folks were turning them inside out to use for first class and such. so, regardless of whether I agree, things have evolved to a specific state of pricing and usage. if you want to use a "flat rate" box for a different price tier of Priority, turn it inside out. some folks get away with just covering the designation on the outside of the box.
yes, it is very convoluted! I have been selling for about 20 years and it seems every year they change something and the prices just keep going up.
you can get slight discounts by printing labels and paying through PayPal.com/shipnow .
The strange thing is you can get free Priority mail boxes of the same size. I was charged more because the box I sent had flat rate written on it, which I had covered. I will not do that again. I do use the Pay Pal shipping site that you supplied to us in the past. Thanks Al.