[Who is John Galt?]

It was all I could do to contain myself from getting a sharpie and crossing out “priority express’ on the box.

Crossing out the labels might work, IF you ALSO cross out all of the bar codes printed on the box. But if you're trying to use a certain type of service like flat rate Priority Mail, you may be required to use the appropriate/specified packaging, because the rates are based on the specific package characteristics (including volume), and the pre-printed bar codes are used for billing and may influence automated sorting.

Sometimes frustrating (especially if you have a busy PO & have to stand in line), but I generally prefer USPS; in my experience, FedEx is more expensive, while UPS is more expensive AND destroys things...

[AbolishCSEU]

there were no old labels nor bar codes. Completely unused priority express box.

Again, government employees get their reputation of being bottom of the barrel honestly.

[brooklin]

“UPS is expensive and destroys things”

But as a courtesy, at no extra charge...