My mom is planning my shower, but she asked me to help with the invitation. I need y'all to double check my grammar please.

At the bottom, I'll be putting the registry info but I can't figure out which way it needs to be written, with or without the comma after Amazon:
A:
Registered at Target, Amazon, and Babies R Us
B:
Registered at Target, Amazon and Babies R Us
Re: Calling all my grammar girls
I need to send this to my boss!!!! She doesn't like the oxford coma and it drives me crazy.
Also this is why I like Math... rules are rules!
For what it's worth, when I teach writing lists, I have my students put commas in between each word, including before the and.