We are arriving in Cancun (me and my girlfriend) Saturday 7.17 pm (the flight from Atlanta), and we have to get the same night to Tulum. Our hotel is just at the entrance to ruins. The most reliable way to do it would be to arrange cancun airport - cancun transportation with one of the companies. You can have a private van for 110 up to 150 dollars, what makes it quite expensive for 2 persons but really affoprdable for e.g. 10 persons. An alternative is 35 dollars per person in a shared van which makes it for 2 persons almost as expensive as a taxi.
Perhaps somebody would be interested to get from Cancun to Tulum on the same day? If there would be enough people than the private van could be booked and it would become much better deal.
I have to decide on the van till Wednesday as all these companies want to book 3 days in advance.