Upvote:3
I experienced this when travelling in Mexico a year and a half ago. ADO's site simply will fail at processing non-Mexican payments. They are far from the only Mexican website that is very cautious with this: there is a very high rate of payment fraud in Mexico. I've even had my card blocked trying to book flights on Mexican airlines' sites for travel from the States and Canada.
Luckily, I happen to work at Busbud and we've since been able partner up with ADO to offer many routes and accept international cards (and charge your card in your home currency if you prefer).
If we don't offer the route(s) you're trying to book let us know and we'll see what we can do.