On your actual line item invoice, once you've sent them an invoice that combines the shipping to $5 and they've paid it, you'd be charged the fees on each auction as though you'd charged 50 cents shipping for each item, so the 10% fee would be 5 cents on each of the 10 transactions, making it 50 cents total, 10% of the total $5 charged, as noted above.
I just re-answered in a bit more detailed way just to point out this is why it's important the buyer pay on a single invoice for multiple items whenever possible. If they buy 10 items, all with $4 shipping, and pay for each separately, your Ebay fees on shipping would be for the $40 total shipping they paid, or $4. You would usually refund the extra shipping they paid as a Paypal refund, $35 in this case, but Ebay doesn't return the fees and keep only the 50 cents on the $5 shipping you actually charged, they keep the whole $4 on the $40, so you'd be out the $3.50 in that case.
It doesn't happen too often, but it does happens, and you don't really want to be mad at or else nickel-and-dime someone who just bought ten of your items, so in those case, I usually just cover the expense out of my end, and be glad for the purchases. I might send a friendly note letting them know why it's better for us sellers if they wait for a combined invoice before paying if buying multiple items in the future.
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