Seller: How can you give free shipping to a buyer?

So I have a buyer who contacted me and asked if they could add some more items to an order that has not yet been shipped. I'd prefer to let this run through Bricklinks system as well, because this would make handling it for me easier.

But I'm struggling to find a way to give this user free shipping without opening it up for all users. So far I've tried:

  • Issue a coupon for the shipping costs, this didn't work because a coupon is limited by the value of the order and because my buyer wants to add items for less than the shipping costs they would only get the value of the items for free but still would have to pay the full shipping costs.

  • Create a "Free Shipping Method" with a bypass password. This didn't work because the password seems to only lift the global minimum buy restriction, but no shipping methods are displayed during checkout.

Do you have any suggestions on how to create a free shipping?