How can I change postage amount after a sale?

[jillcopago]

I am trying to combine postage for a customer who won more than one of my auctions. ON the "send payment info" page there is places to change it or for Seller discounts (-) or charges (+):

but neither of them 'take' what I try to put in and want to keep charging the original amount.

How do I do it?

many thanks.