Neighbourhood seller

Like clockwork, on Friday at 830am this man and his lorry comes around the neighbourhood with his horn going beep beep beep.
He sells cooking gas in a ready to use tank for a few RM (ringgits) more than the shop and the convenience.

How does he knows you need a new tank?
You remotely open your gate. He spots the opening gate. He takes a full tank from the back his lorry and drop it off to your previously nominate spot. Swap the empty tank for you. Pay him and Done.

Other sellers in the neighbourhood are paper collector, ice-cream man, stickman (broom, clothes hanger, bamboo stick), gasman and ?postman (beep beep).