Checked bag on Delta

[leebee]

New to flying Delta; I usually fly Southwest, but finally had enough Delta points for a free flight! My ticket is a basic fare and I will be checking a suitcase. I have a Delta AmEx credit card so my suitcase should be free- but how does this work? When do I "pay" for my bag- when I check in or at the airport? Will I not be charged at all, or am I charged and a credit posts... or what? I have one more SW trip with 2 free bags and will really miss this benefit.