[high_density]

...but you can delivery boxes...?

so put mail in boxes?

I mean, there's DHL, Fedex, and others...

[gumby]

I don't believe the law has ever been revoked that requires anyone sending documents through a non-USPS service* to include the appropriate USPS postage cancelled with a pen along with their shipment

* excepting a point-to-point courier service for some reason

[yunesj]

The USPS carved out a few exceptions, like delivering urgent mail. Gaming the system will get you raided by armed agents of the enforcement arm of the USPS.