Here are a few things I found out recently about the Prepago SiM cards from ICE and a recent change.
ICE now just changed the time on all amounts (2500, 5000, & 10000) to be sixty days
ICE only wants the Prepago SIMs to be used by tourists and supposedly won't sell them to residents, this appears why they are only available at the airport booths. Since the booths are in the secured areas of the airport you can't reach them if you left the airport until you are ready to fly home. Trying to get a Prepago SIM at a local office normally just leads to being told they have none, they don't sell them, or they are only available at the airport. If you need one, are not resident go to the ICE office on 5th ave. you should be able to get one by reminding the manager that you can't get back to the offices at the airport as they are in the secured area. I might also say something like I purchased a phone in town that could be used here, and therefore couldn't get the SIM at the airport as I didn't have the phone. They do have them at this office, it is just asking nicely and getting a manager to tell an agent they can do it.
Lastly you can now buy the recharge codes online at masterpago.com with a credit card. You will get a 14 digit code to enter into the phone just like you got with the refill card from the market. You or at least the phone needs to be in CRto enter the recharge code. Still done by dialing 150 option , then enter the code.
I had trouble on the 24th with the site, got no code but they were easy to contact, spoke english, realized there was a problem with the site and are resolving the problem.
Bill