Combined shipping does not mean "no additional shipping charge for extra lots." It means that the seller will combine the items into the same packet, which will generally yield a discount on shipping because it is cheaper to ship 10 ounces than it is to ship two 5-ounce parcels.

Sellers that offer $0 for additional lots tend to express that in their listings.

I have been selling on Ebay for many years and usually sell 5-10 packets of inexpensive coins per week. I offer combined shipping at an extra 10 cents per lot, so if someone buys 4 lots they pay the base shipping rate plus 3 x 10 cents. This has worked well for my customers - they save money, and I have never had one take the position you are urging now.