One full time dealer who sells on eBay
as well, listed the opening for all lots as $0.99 plus shipping. Buy multiple lots and the shipping would be combined and adjusted. Then eBay
pressed for "free" shipping from sellers. Eventually the dealer went with "free shipping" on each listed lot but raised the openings to $2.99 for every listing. Buyers still purchased many items together, but because shipping was "free" they got no shipping price break but the dealer made more money on the built in cost of "free shipping" at $2 per listing.
Me, the built in "free" shipping is wise but screws the buyer; however, to minimize the screwing, after so many purchases a dealer adjustment (discount) could be applied to drop the cost of shipping on the buyer. Such as a dollar per listing purchased beginning with the third item makes the buyer feel better and the dealer able to cover the minimum expense of sending out one or two small (inexpensive) lots.
But the bottom line in business is there is no such thing as "free."