You can now buy public transportation tickets online with the RET Barcode app. The app is available for both Android and iOS devices.
Public transportation in Rotterdam is usually a breeze. All you need is one card and just like magic, you simply check in and out of all the modes of transportation. But the ov-chipcard system also has one major flaw; when you run out of credit, you can't just re-up anywhere. Of course you can always purchase a ticket if you're carrying cash, but fewer and fewer people carry cash nowadays.
The downside
If you live in the suburbs, you probably need to take a tram to get to a station where you can finally purchase the credit. This often involves an uncomfortable conversation with the conductor in which you plead to please let you ride along for a few stops. If they kick you out, you waste a lot of time finding an ATM and making it back to the stop. By then you probably missed one or two trams and are now late for your appointment. To make a long story short; there needs to be an easier way.
RET Barcode app
RET has recently introduced the RET Barcode app. If you need to catch a tram or bus, but don't have any cash on you (or on your card), then simply purchase credit through the app. You'll then receive a barcode that you can show the conductors when they come to check. Just make sure to activate your ticket within 24 hours after purchase.
At present time, the barcode in the app is not yet compatible with the gates at metro stations. However, it is predicted that this will be the case by the end of the year. By the end of 2019, you'll also be able to check in & out of trams and busses by scanning your phone (instead of having it inspected manually by the conductor/driver).