Order payment query

[EmmaJanesGifts]

Hi,
Please could you advise on the best way to add an extra payment to an order that's already been paid by the customer. They purchased 3 items and paid for them but added that they would like something else as well. I've not had this before so don't know how to deal with it. My first thought was to send a separate PayPal invoice for the item, as they won't need to pay the P&P again as it would already be covered by the order they've paid for.
Thanks, in advance, for any advice.

[Former_Member]

I would create a custom listing with free shipping for them to purchase.

[StuckOnSilver]

I tell the customer to buy the item listed and I will refund the excess shipping charge. Creating a custom listing works, too.