EBay won't let me remove postage from invoice

[darkdragonempress]

I have just sold two items to a buyer and she is going to pick them up in the next few days. She wants to pay with PayPal and I am trying to send her an invoice with postage removeed, but every time I preview the invoice and select "Local pickup" and put "0.00" in the postage box, and then click recalculate, it just keeps resetting itself. I have also tried just putting "-21.80" in the Seller Discounts or Charges box, but again, every time I click "Recalculate" it resets and shows the full cost of postage as well as the total for the two items. How can I remove the postage? I am sure this used to work when I did it in the past so I don't know if this is a glitch or if they've changed it so you have do some other convoluted rubbish just to get it to behave.