We've been to Cancun several (over a dozen) times.
We've used USA Transfers, Canada Transfers and Sarmez shuttles all with great success. They have good websites and you can get your quotes there.
On our last 3-4 trips, we've used Sarmez, because their rates were the best. Their drivers are always prompt (on time or early), courteous and drive well. On our most recent trip (just 3 weeks ago) our return flight was 10:30am and the shuttle service recommended a pick up time of 7am (This was leaving from Cancun Hotel Zone, not Riviera Maya, by the way and we paid $65 round-trip)
I thought that was too early, but actually, it was perfect. We got the airport early, but the lines were insanely long.
You can check each website for their rates:
USA Transfers:
https://www.usa-transfers.com/Canada Transfers:
https://www.canadatransfers.com/Sarmez Shuttle:
https://www.sarmezcancunshuttle.com/en/