Can you send an invoice to someone without having to list the item?

someone wants to buy a multiple of something I have listed

thought the easiest way would be to just send them an invoice, but this doesn't seem possible

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No because then eBay won't get their fees.

Do a separate Buy It Now listing for the quantity they want and the agreed price, then contact the buyer and let them know the item number so they can buy it before somebody else does.