Did the buyer buy multiple items? If the original listings were for $3 postage per item and the buyer has purchased 3 items then they would expect to only pay $9 postage. If you have changed it to $11.50 due to the increased weight then in checkout they can select to uncombine the invoice and save themselves the $2.50 by paying separately. If that is what's happened then I'd post them separately.

I think it was the "GARY ABLETT - IRONBARK LEGENDS " with postage of $11.50.  Do you actually remember the $11.50 showing when you were posting the invoice?