Letter is the cup size. Size of the boob itself.
Number is the chest circumference.
Willing to buy her undergarments, but not willing to ask the hired help basic questions?
Anybody at Victoria's Secret would be more than willing to help you with that information.
I always feel like a panty thief in that store. 5 for $30... so many choices.
Honestly, I just get her a little pack of little boys super hero undies... that does it for me. I know... something is wrong with me.