First 'Free" Shipping just means that the cost of shipping is included in the price.
So if your $5 item required $10 to ship, you advertise it as $15 with Free Shipping.
Now if you sold three $5 items as $15 with Free Shipping, your customer would be paying you $45 which includes $30 in shipping costs.
Are we still together on that?
Some people have difficulty with the concept that 'Free' shipping, isn't.
If it actually costs $30 to ship your three items together, no problem.
But if the three items together only cost you $15 to ship (and that includes packaging, bubblewrap ain't free), then you are taking an extra $15 profit.
Are you okay with that?
Will your customer be okay with that?
If so, no problem.
If not, in my opinion the best way to handle the problem is to refund an appropriate amount using Paypal's Send Money service.
An appropriate amount, would be less than the $30 'overcharge' because doing this will not remove the eBay fees on the sale, so you are , again in my opinion, entitled to take that cost from the refund you send.
Most people are delighted to get any refund, especially out of the blue.
Technically, of course, you told the customer what he had to pay, and he has little or no idea how much you actually paid to your shipper. So he should not be complaining.
