How to adjust the price to include shipping after the item has been paid for?

[option2020]

Purchased a large item and have closed the deal (3 days ago) but we are now trying to ship the item.  The shipping charges have now been agreed to by both parties but how does the buyer pay for the extra shipping cost?

[momcqueen]

Strictly speaking, a seller cannot ask the buyer to pay more for shipping than was specified in the listing.