postage difference

[greenwardrobe1955]

I sold an item to a customer who then informs me she is bidding on another yet pays for the first item prior to the second auction finishing then asks can they have combined postage no problem say i i will send invoice deducting difference which i do she then informs me the invoice is incorrect and that its showing the full postage rate £3.95 so i refund the other items postage to level it up again for her to then pay for the second item however the postage is at the lower rate with the difference taken out and not as she claimed at full rate Does not seem to be any way i can amend invoices via ebay so have stimulated a invoice for the postage difference via pay pal advising her to pay it then i will send items out am i ok to do this and is there a way i can address this via ebay?

Don't want to be out of pocket on the postage

thanks