buying nightbus tickets Palenque to Cancun

You should be able to buy tickets at any ADO terminal.

If you want to buy on-line, try the "ClickBus" website -- foreign cards are accepted.

Yes, you can buy any ADO route tickets at an ADO counter.

And if you buy in advance, you`re more likely to get a discount.

Thank you all!

Yes, correct info above. I have bought multiple tickets at one time, many times. What I do is write all the info on a piece of paper for each trip, with both the day of the week and date (and time) noted. That way there is less chance of any errors. The clerks are always very careful and confirm everything. You will be asked what seat you would like and they require your name.

I use my credit card and it usually works at ADO, though not at the Cancun airport.

Using Clickbus is more expensive, as they add fees and do not offer the advance discounts.

Thank you, that is very helpful and I will do so!
One last question: When I took ADO from Mexico City to Oaxaca I am sure is said ADO GL on the website and I thought I had booked a ticket on the Platinum Line. However the bus was an Ejecutivo class.
The nightbuses from Palenque to Cancun are Primera class and the later one Ejecutivo class. Is the Primera one the better class?

I just looked at the ADO site and the bus at 23.40 (just before midnight) is a GL bus. This is a step up from the regular first class, but higher yet is Platino service, which I do not see offered on that route. Platino seats are larger (only 3 across) and recline really well.

For me, regular first class is just fine. I have travelled twice on the GL service as that was the bus at the time I wanted. Very comfortable and I slept well. The bus will be quiet.

I checked for tonight, and besides the second class AU departure, they offer two ordinary first class departures (OCC and ADO), and the gl or gran lujo already mentioned. OCC and ADO are both first class marks and more or less equivalent.

For the example of tonight, the first class buses are discounted, but not the second class, so not even worth considering that unless the time of departure is more convenient.

Thanks very much! I think I will take the earlier one though, it leaves at 21:40. Might be less comfortable but I think I will like to get going after visiting the ruins in the afternoon.
Thanks again for being so helpful!