Apparently a medium flat rate box got in with the free boxes and I just didn't realize while I was packaging it. Anyway, I noticed up at the counter, and this is the exchange I had with the woman helping me.
Me: Oh, I didn't realize I put this in a flat rate box, I just need to go repack this.
Her: Uh, you're just going to waste my box? Just going to throw it away or what?
Me: It must have fallen in with the free ones, I don't want to ship this flat rate.
Her: It's already in a flat rate box, and I'm already at the counter, so that's how you need to send it.
Me: The hell I need to. I'm just going to repackage this, I'll be back in a moment.
Ok, maybe I am not completely innocent here, afterall, I did mistakenly grab the wrong box, but it was in the wrong place to begin with, and her attitude was shit from the second I walked up there.
So, I repacked my box, threw the wasted one in the recycling bin and made my way back up to the counter.
Her: All set now? Figure it all out? (in a mocking, antagonizing tone)
Me: Yeah, it's fine.
Her (to other postwoman that happened to walk up): This is the one that's wasting all our flat rate boxes.
Me: It was mixed in with the free boxes, I grabbed it by mistake.
Her: I bet. Your total is $5.xx.
The rest of the transaction was unremarkable, but really now, what the fuck is your problem?