Do I need to print a shipping label before going to the post office?

I feel like a total idiot for having to ask this, but I’ve only ever shipped packages with USPS’s click and ship where you buy the label online, print it out yourself, and then schedule for a carrier to pick it up from your front door. I’m shipping something that can’t sit out in the cold (live plants) and so I want to bring the box to the post office to ship. Do I have to print out a label and/or pay ahead of time or do they do that for me at the post office? I’m using a priority mail flat rate box.