Speaking from my own experience of booking GrabCars (not GrabTaxis) directly through the app (not through their airport booth), I can attest that drivers do have the option to manually add tolls to your fare.

Motorway operators are absolutely keen on getting drivers to use RFID stickers in place of cash. More and more toll barriers are refusing to accept cash at all (especially along the SLEX); for example, at the exit I frequently pass through, cars without RFIDs are made to pull aside so that they can get fitted.

I'm not the trusting sort of chap so when taking a Grab fitted with an RFID and/or passing through a stretch of road that's gone cashless, I'd typically ask the driver NOT to add tolls to the fare. I'd rather reimburse them for the tolls, in cash, at the end of the trip so that the only charge I'd see on the app is the actual fare that was initially shown on booking. (Note: I use GrabCars, not GrabTaxis, so the fare shown in the initial search is all I'm charged; there's no variable component save for tolls as these cars don't use metres.) I did let the driver add the toll manually on one particular journey and I made sure to agree the actual toll amount against the amount he added, but I normally find it simpler to just tell them not to do it.

Cheers.