I purchased a ticket on Volaris, and I noticed a section under the fees titled Multiple Currency Exchange Service and Multiple Currency Exchange Service Tax. On a $106.43 ticket this was $5.32 (5%) plus an $0.85 tax for a total of $112.60. Changing the currency at the top of the screen to MXN resulted in a total ticket price of MX$2188 or ~US$106.

Upon entering my payment card information (Visa) the website once again asked if I wanted to pay in USD for $112.60. The MXN option indicated that it does not include the exchange rate fee. The USD option said I could save up to 4%. I don't know if I follow that math since paying in USD was about 6% more expensive.