We read about Una Casita on TripAdvisor so we decided to check it out. We went in the afternoon just to make sure we knew where it was. We talked with Elly, the owner, and told him we would be back later for dinner, but when we went there in the evening we found the food to be not quite as we were expecting. When we paid, it was 25 quetzals each (just over $3), and he explained to us that he didn't prepare for a better meal because people come all the time and say they'll come for dinner, but never show up. I wasn't flustered by this because the meal was still alright and he gave us a good price.
So we made reservations for an exact time for the next day, and we were very impressed by the food. We went there the next night too! So all you have to do is tell him you'll be there at a certain time and he will be ready for you.
FYI - bring some bug spray. Also, a little extra money just in case you want to buy some of his crafts he has for sale there.
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