eBay Invoice Voided Postage Costs

[fallen121]

A buyer bought a substantial number of books from me. As postage was going to be £14, they asked for a invoice to be sent. I completed an inoice for all the purchases, and reduced the postage cost to £2.99. I checked the invoice prior to sending.

When the Paypal payment came through, I checked the  invoice they had paid and postage was showing as zero. The £2.99 postage is shown as an "option" and the buyer had selected "collection in person - free" which is clearly nonsense as they live in Middlesex and I live in Scotland.

 I am now expected to send 7 books for free and as the books were substantially discounted to start with and I will pay eBay and Paypal fees, I stand to make almost nothing on this sale.

I checked and re-checked this invoice. What in God's name did I do wrong?