I've had a similar experience when I used to be a cashier. However, in my case, this person was looked like she was starting out and people in store noticed. She was wearing female clothes and had a pocketbook, but she still had boyish appearance, short hair, questionable voice, and her walk was overly animated. I honestly believe this was her first time out because she seemed incredibly nervous when she came up to be rung. However, I greeted her as female and did not show any sense that I noticed anything. I made small talk with her and acted like she was no different than any other female customer. All of a sudden, her whole attitude changed. I could see she was excited to feel like she was passing and she started to smile. It made me feel real happy because she went from looking really tense while in store to leaving with a huge confidence boast.
Unfortunately, this moment was ruined when my coworker looked over at me with a weird expression and said "jesus, lots of those weirdos nowadays". She was out of the store by then, but me the secretly closeted transwoman got to hear my coworkers bigotry towards people like us. It wasn't enough that she had people staring at her while shopping, they had to make a nasty remark behind her back.
I'm really glad you did that Maidoforleans. Even if you were overly animated, she probably didn't notice you detected it. Most likely she felt like she passed and felt very excited about it. Seriously, I'm betting it meant a lot to her.