Talk

I know I've done this before, but can't find right button to press!
I've just had one buyer for 4 separate items, she has paid me for 3 of them, but I've mailed her and told her not to send me the payment yet for the fourth, as I was pretty sure putting them all together, we could save on postage...this is so, have weighed it, so I now want to invoice her, but remove the £3.20 postage for the item...I've pressed recalculate and tried to remove the postage, but it won't let me...What am I doing wrong?