I just used cancuntransfers.com for transfer between cancun and Chichen. Reasonable rates, met me at the airport, excellent service. Yes, you need to bring your own carseat.

Use USA Transfers

http://cancuninclusivevip.com/cancun-airport-transfers.html

If you booked a package deal, your transportation may already be inlcuded. Check that first.
If not, we always use Cancun Valet. Do NOT go with anyone inside the airport after you go through customs. They will all tell you they are your ride. They are time share sales paople. Your transportation representative will wait right OUTSIDE the door from the airport.

I've used USA Transfers 3 times, they are great, $130 per van RT to Akumal, probably a bit less to Puerto Aventuras. I believe they do provide car seats but cannot guarantee them so you might want to bring your own.

The Cancun airport is quite an experience. Do fill out the entrance form correctly, do not miss a line or they will yell at you and move you to the side. The luggage is x-rayed again before you LEAVE the airport. Long lines...bring your stroller. Then you pass thru a gauntlet of salespeople in blue shirts..Do NOT ask them any questions or you will get stuck with them.
We had shared transportation from our hotel. Big mistake. We waited in the hot sun until they filled the van. Do have private transportation. USA Transfers seemed to have lots of vehicles and was efficient.

Fantastic suggestions - thank you!

I think, http://cancuninclusivevip.com/cancun-airport-transfers.html ...this is best for you.
regards,
http://www.suainlogistics.com/

Does anyone have a phone number for USA Transfers? I want to ask them a question but can't seem to find a number of the website referenced by several people above. Thank you!

I would just email them with your question, that is normally how they prefer to do business and they usually get back to you the same day. When you book, you will get two emails, one for transfer to hotel with the amount due and the second for your return to the airport showing zero balance. Print them both and take them with you. If you prefer, you can also pay in advance using PayPal, but you have to email them for this option and then they will send you a link to their PayPal account. I have paid cash, last time paid in advance.

You will get a phone number to call them if your flight is cancelled after you book.

Anyone know anything about Olympus Tours? I booked my vacation package through cheapcaribbean.com and that is who they offer transportation through. It's definitely cheaper, but I don't know if anyone has experience with them. Thanks for all of your help!

I would just email them with your question, that is normally how they prefer to do business and they usually get back to you the same day. When you book, you will get two emails, one for transfer to hotel with the amount due and the second for your return to the airport showing zero balance.

http://www.navisworksfreedom.com/

I just thought I would follow-up to say that we ended up using Olympus Tours for our airport transportation and were very happy with them. This is the company that partners with cheapcaribbean. We booked directly through cheapcaribbean because it was less expensive. We booked a shared shuttle. On the way to the hotel, my husband and I shared a shuttle with two other couples, so there were three stops total. Unfortunately, we were the last stop, but the additional stops probably only added 20 minutes to the ride (we were going all the way to Puerto Aventuras). For the cost-savings compared to the private shuttle, it was totally worth it. On the back back to the airport, we actually were a party of four and were the only group in the shuttle, so we went straight there - awesome!
I would certainly recommend them. The staff was very friendly and offered us bottle water as well.