I think your best option is to book the transfers ahead if time. If you wait until you are in Havana, you are more likely to pay much more, plus you will have to spend part of your trip trying to book the transfers. I don’t know if you speak Spanish or not, but unless you are a good Spanish speaker, you are likely to get taken advantage of.
Taxi transfers are not cheap. Even when booking online, a transfer like this will cost a few hundred dollars. Dealing with a website such as ours, you have some degree of certainty with your reservation. Last minute issues can always occur with any reservation, but fundamentally we deal with dozens of clients per day and have a good reputation.
If you just deal with a random taxi driver in Havana, you never know what you are getting.
If you are interested in booking a taxi transfer ahead of time, you can shop around online. I know our prices are likely to be the cheapest. And if you want to reserve with us, just use the link at the bottom of the following website: http://bestcubaguide.com/city-city-taxi-transfers-cuba/