With practice it will get better. And I bring candy with me as a thank you. One time I had an old woman do a New York style bait and switch. Pissed me off. But I just chalked it up to experience. And it was a $3 hustle... I don't think 50 cents would have bothered me as much....
You could just shop at SuperMaxi or Coral. Not to many problems there, just more expensive. But their fruit last longer too. The stuff at the Mercados seem to be ripe and needs to be used by tomorrow.
Practice your Spanish and don't be afraid to walk down to the next gal or guy. I don't think that is rude, I think it is expected.
Good luck....