Hi Richard,
Thanks for posting. No there’s no online system for that, you can add money to your metro card at metro stations and then hop on. The last time I was in Tocumen airport unfortunately there was no way to buy cards or put money on your card, which is silly, I know. But you can walk out to the bus stop and offer a local money to swipe their card for you. If there’s no one waiting for the bus then wait till the bus gets there and climb onboard and ask the passengers onboard to help you out.
For any others reading this post,just fyi, there are also taxis at the airport, car rentals, and you can call services like Uber or drivers listed in the travel/rolodex portions of this website…so plenty of options.
Happy travels!
Best,