They've gotten tricky with the add-on gimmick ..The plugs i wanted at a great price, plus wires put me at $35.25,NOGO.That auto tagged the plugs as add-on item.
Rockauto got the order instead,for $31,including shipping.
The price history of that item shows that it was just over $35 not long ago. The price setting algorithm that they use currently has the item at just under $35, but that can fluctuate depending on demand, cost, competition pricing and a host of other factors that are taken into account.
This item happens to be at a point that fluctuates both above and below the $35 free shipping line. I'm sure if you search Amazon you'll find many other items that do the same. As well as many items that fluctuate below and above $40, below and above $50, and so forth.