Re checkout and two or more items.

Hi

Ive been selling on Ebay for 2 months now....Ive just had an Ebayer want to buy 2 items - however wanted me to change the invoice for the postage. I told her to buy them then I would change the invoice before paying. She said when she purchased the first item it took her straight to paypal...to pay for the item.  she ended up paying the full postage because we couldnt work it out-then I refunded her the difference. Is this normal....or is their a tab - continue shopping Etc....appreciate any feedback at all......Thanks Jo 🙂