The roundtrip price for one person was $35. We will assume a couple which adds up to $70 USD for two buying in advance.
If you get two tickets for the shared van it is 78 pesos each so I will call that $14 USD for two to get to El Coz from the airport.
The return taxi...let's be cautious and say $15 USD for two.
$14 + $15 = $29 USD versus the package add-on of $70. Sure it may take you two minutes to buy the tickets, but I say that is worth it to save $41. Just my opinion.