You can give a UPS driver a package that has the postage paid on it.
For example if you have a package that you're shipping then you can print out the shipping label and pay for the UPS postage.
Then when the UPS driver comes to your door to deliver other packages or you see a UPS driver nearby you can hand them the package and they can then scan it and take it back to the UPS location.
Or you can do a return package that way as well.
Also if you have a UPS store nearby you can drop off the UPS package at the UPS store and then it will be shipped out.
I've handed a UPS driver a package and they've always taken it.