Absolutely horrible experience, very rude counter people that will go to ridiculous extremes to try and ruin your check in. I purchased their instrument carry on fee prior to my trip and when I got there they said I had to purchase a seat for my guitar. Standard case with standard soft case measurements and they measure it twice to try and say it was too big and I would be charged extra. I had to show them how to use a tape measure. Cheap flights to mexico yes, and their customer service counter people definitely reflect the discounted ticket price.
Do NOT fly Volaris.