If it's after the fact, meaning you created a shipping label and paid for it, now you want to add to it, unfortunately you can't do that thru PayPal. You would need to go to the post office.
Welcome to the Community Forum! If you made the original payment through PayPal, I'd recommend asking the seller to refund it, and you can re-send it to them including the additional shipment cost amount. Alternatively, can send an additional payment to the seller, but this won't be covered if you need to open a dispute for the payment. Hope this helps!
