So yes I snapped a few photos of it. Seems like the postal buggy / van will roam Terminal 1 & 3 of Changi Airport at different timings. Wonder if they have any special cancellations for the stamps purchase at the venue. :)
The funny thing about this whole postal setup is that there is actually a mascot for it. The mascot is a postbox modelled after the old style circular / tubular type, with a name of 'PoWee' I think. I am guessing that PoWee stands for Post on Wheels.