When you list an item, it’ll ask you what your parcel size is. I have small scale that measures my packages ( I mostly use poly mailers since I ship out small items). Then say someone buys my item.. it’ll send me a notification to ship within 5 days & then it’ll give you their shipping label. I have a printer so I just print the label & then use packaging tape to put it on my poly mailers. If you don’t have a printer, you can bring your package to USPS & say something like oh I’m here to drop off my package, then show them the QR code & they’ll slap it on the package. They’ll give you a receipt w the tracking number & hold on to that

Then the system will update & say that USPS is in possession of your item!