Some post offices may have a label printer in shop but it’s not needed to send a parcel x
They can bring a label to you If you pay online for them to pick it up
The post office or the postie can print you a label. You just have to choose that option when you pay for postage and collection
Write the address & return address and stick it to the parcel, take it to the post office and ask to post, they give you proof of postage receipt and that’s what you take a photo of and provide the buyer with. Hope this helps x