You seem to confuse the words "buyer" and "seller". I guess you mean to say
"Buyer purchased an item and paid for it including the shipping charge. Later, buyer purchased a second item."
And you now want to combine both transactions into one.
Sorry it cannot be done. What you need to do in send an invoice with ZERO shipping charge which the buyer will pay.
The result: two purchases, two payments, one shipping charge.
"This is wrong!!!"
It is nobody's fault. It would have been preferable if the buyer had not paid so quickly, allowing yu to send an invoice combining both items with only one shipping charge.
