Maybe this will help? https://talpamascota.com
ATM has its own terminal in the hotel zone, in Versalles, off the main highway. Here is an English page on its website. When I used it a few years ago, you had to buy your ticket at the terminal. They told me to just show up a bit before the departure. I purchased the ticket from a young lady who spoke no English sitting at a card table at the side of the bus yard. It is a "rustic" setup, so no surprise that there is no internet ticket purchase. The buses are NOT luxury vehicles such as you might take to Guadalajara, but they seem safe. The ride itself is very scenic.
