You're right, having a credit card declined is not necessarily embarrassing. What dd did was...
"customer's credit card had been declined and I shouted it out bc I didn't know what to do. "
THAT would be Very Embarrassing.
Remember the scene in Terms of Endearment when John Lithgow comes to the rescue of Debra Winger?
It was right after the cashier yells out "CAN I GET A MANAGER OVER HERE? SHE DOESN'T HAVE ENOUGH MONEY."
Sure your card can be declined for other reasons, but the other people within earshot don't know that.
I am absolutely sure that if I was standing in line behind someone, and the cashier yelled out that the customers credit card was declined, I would immediately say "I cannot BELIEVE you just did that."