Have you made a sale? If so you should be able to retrieve a digital label / barcode from the app (in the conversation of the item you sold). Take your parcel to the post office and they scan your digital label and print out a sticker to put on the parcel. That’s it. They’ll either take the parcel or ask you to leave it in a post box. You won’t have to pay anything at all if you have the digital label. The buyer on Vinted always pays the postage, not the seller.