My wife ships with endicia and shopify. I've often wondered if the barcodes that are printed are actually tracked, as you can void them. With that void comes a "i'm not going to ship this anymore' self-certification.

I suspect that the postage is actually generic and can be reused, but for obvious reasons don't want to try.

You can also take postage paid envelopes and stick them on anything. People used to stick them to bricks back in the day just for fun.