You can top up using online banking if you have a Dominican bank account. From Banco Popular it's really easy, you just set up your mobile account under the Communications tab and you can top up from there. I would also strongly recommend you check out the cost of getting a contract with Claro instead of using Pay as you Go services, even if you are not here often. Their new contracts let you carry over minutes you don't use from month to month, so work well for people who are not here some of the time. And they also include free calls to Claro cellphones and landlines.