So I cancelled the order and re-listed with the correct delivery cost.

It was a bit confusing as the larger package size is no longer eligible for Simple Delivery which means that the buyer still pays postage but I also had to pay for a new postage label to ship the item. EBay have said that the buyers postage together with the selling price will be added to my account in a few days so my shipping costs will be covered.

Hopefully all good in the end but it would be so much easier if the seller was given the option to cover any additional shipping costs. It was a genuine mistake. 

all good for now though. thanks