Here, I am assuming that you do not have someone that can drive you to the Reynosa REX airport and that you are like me, wanting to catch a low cost flight from the Mexican side of McAllen - City of Reynosa.
It is best to catch a bus from the McAllen Bus Terminal for $5 to the Reynosa Bus Terminal or have someone drop you off a the Hidalgo International Bridge and walk across. Either way, once you reach the Mexican side of the bridge, their Customs will scan your baggage for contraband, especially, guns. If they find one or ammunition, enjoy a life in their prison. They are no-nonsense about it.
If you take the bus, you will be asked to get off the bus and couple of helpers will take your luggage to the scanning area and then help load it back on the bus. $1 tip per bag is decent. Then, it is a 5 minute ride to the bus terminal. Depending on the hour of the day or night, take a taxi from the bus terminal to the airport - $MX 200 or Uber for less. Same process for the return journey.
I do not recommend buying the bus pass offered by VivaAerobus when you book a ticket online as depending on the time of your flight, it may not be practical and the wait for the shuttle can be long. Their bus pass runs about $MX 130 from McAllen to Reynosa airport, for about $MX 180 more, you can travel on your schedule cutting your time from 3 hours to less than an hour.
Same in Guadalajara - instead of buying the VivaAerobus pass online for $MX130, as soon as you exit the terminal, to your right, catch the VivaAerobus bus and pay $MX70 to the driver.
THERE ARE NO VISAS REQUIRED for travel into Mexico. In all these years of travel, not a soul has asked for any such documents. When boarding flights, only a government issued photo ID - DL or Passport is good. Upon return and flying into Reynosa, they do check your immigration status at the airport, by the Mexican Immigration. I show my US Passport and all is good. Not sure why they check upon coming into Reynosa but, not when you travel to any other place in Mexico. When traveling by bus, no checks whatsoever on the Mexican side. Only when you cross into the US, US Border Patrol requires a passport or a birth certificate and a photo ID and in that instance, a strong suggestion that you get a passport.
As of this blog post, the currency exchange rate was $1 = 18.80 pesos.