Hopefully , they have not paid so you can send them a revised invoice.
Go right of the item sold, to More Actions and select Send Payment Details.
Amend the postage to 0.00 but do not click on reset.
Send to the buyer.
Cash is best for collection , so message them now they have purchased to pay with cash after inspection and collection.
You then go back to More Actions and select Mark as Paid and Mark as Dispatched.