"with no promo"
Do you mean you paid full listing fees for all your auction listings?
I checked one of the auction listings and it states "no Combined shipping on Auctions"
Why? Does it not make sense to offer combined shipping so buyers buy more than one item from you at the same time?
"did that to make is so the buyers didn't pay tax on the initial $3.99 "
???
Not sure what you mean. If shipping to the USA, American buyers do not pay tax in any case. If shipping to Canada, the shipping and handling charge ($3.99) is fully taxable at the same rate as the item, depending on the province of residence of the buyer (13% in ON, NL and NB, 15% in NS, etc...).
"I wonder when the day is going to come when a buyer writes me and says hey shipping is not free how much do I owe you"
I have been offering "free shipping" for many years on all my eBay listings and it has happened to me several times.