I had a bad experience with "Cellular Telephone Rentals Costa Rica", aka Istar Media. They kept a $250 deposit in addition to the $195 fee for renting the phone-- beware
As a former resident of Costa Rica, I was happy to see my oldest daughter take an extended trip there in celebration of her recent college graduation. Wanting to stay in touch, I rented her a cell phone through "Cellular Telephone Rentals Costa Rica", or Istar Media ( http://www.cellulartelephonerentals.com/ ) . The fee for the rental, including unlimited in and out going local calls, and unlimited incoming international calls, was $195. They also required a deposit of $250, to be returned once they received the phone and made sure the cost of any outgoing international calls were covered. I made two payments to them via Paypal-- one for the rental fee, one for the deposit, in the first week of January, 2010.
Well, the trip is long over, my emails to customer service go unanswered, and Paypal says it will not mediate a dispute for this kind of service. So, buyer beware-- there are several other cellphone rental options in Costa Rica, and they can't be worse than these crooks. I am hope my getting ripped off and getting the word out spares someone else from the same...