to reduce the automatic shipping charges. I can do that on my invoices to buyers, I just lower the shipping cost and send the buyer that invoice to pay.

The other option is for you to pay the full shipping price and ask the seller to refund the overpayment. I do this for buyers who pay before I am able to change the invoice. (Some sellers refuse to "combine shipping" and lower the price. A little stinky but if that is the case - so be it.)

All of this presumes you will be paying via PayPal. Under the new eBay scheme whereby eBay handles the transaction - I have no idea how a refund can be made.