Why am I so confused about shipping credits?

I can’t wrap my head around it, or I’m doing something completely wrong. I listed a few items as a test and they sold out in the first day (today). So I listed the items at 16.99 and it looks like Amazon had them listed as free shipping. (I had them listed as free shipping).

My question is did I screw up and I’ll be paying for the shipping out of pocket? Do the items based on category receive an automatic shipping credit from Amazon even though it says free shipping?

I tried talking to customer support.. and I got 3 copy and paste generic replies that never even remotely answered the question about shipping credits.