Is it illegal to flip a USPS Priority Mail flat rate box inside out and mail it non-flat-rate Priority Mail?

I think it is common knowledge now that it is illegal to use – even reuse – a USPS Priority Mail Flat Rate Box for shipping methods other than Priority Mail.

In fact, the USPS now prints the following on the inside of their boxes:

“Thank you for using Priority Mail service.”
“Priority Mail postage required.”

No one on the internet seems to have discussed whether or not it is legal to flip these boxes inside out and ship them Priority Mail at the non flat rate.

The sentences printed in the boxes simply say that the box must be shipped Priority Mail, but mentions nothing about the rate.

The reason why these boxes must be mailed Priority Mail is, according to others on the internet, to offset the material costs. Again, no one ever differentiates between flat rate and not when presenting this justification. Instead, they basically say the only thing that matters is that it goes Priority Mail.

It seems probable that the USPS intended to allow their flat rate boxes to be used at the non-flat Priority Mail rate when flipped as language requiring otherwise is conspicuously missing from the print on the inside of these boxes.

If they wanted the flat-rate boxes to be used solely at this rate, wouldn’t they have said so on the inside of the box, since they already went through the trouble of printing ‘must be sent priority mail’ on the inside?

Yes, I realize USPS makes non-flat-rate boxes, but that doesn’t cure my curiosity here. If you only have the flat rate ones at home, or if your post office is out of one or the other, knowing the answer to this question might be helpful.