But seriously, I don't read being asked to help reach a shelf as having been racially mistreated. 
Does it have to be because she was black? Could it possibly be simply because she's tall? Or perhaps the person asking was making a gesture to relate on a "normal" level as you would anyone else in the store. 
I highly doubt when Michelle Obama enters a store to shop, she is mistaken for anyone but the First Lady. 
I saw her in Target once and mistook her for Aunt Esther.  

Then I realized Esther wasn't as ugly as the fish eyed fool I saw in Target.  
These fucking clowns piss me off so badly. 
 I was in Best Buy today.  I wore a blue golf shirt.  Somebody asked me if I knew where the printer cartridges were.  I punched them in the face for being racist.  
One time I tried to get a cab in New York and the cabbie drove right by me.  Fucking racist cab driver.