There are 4 different ways to pay your road tax:

1) You can pay online at www.onemotoring.com.sg through internet banking, OR
2) You can pay at any AXS machine, OR
3) You can pay at Post office, OR
4) At any Vicom inspection centre (Inspection Centers will usually want to see an original CI)

A list of Singapore Post Offices can be found here:
http://www.singpost.com/our-network/post-offices.html?catid=36&id=45

You can find the nearest AXS machine by using this tool:
http://www.axs.com.sg/phpweb/consumer/station_location.php

A list of inspection centers can be found here:
http://www.onemotoring.com.sg/publish/onemotoring/en/lta_information_guidelines/maintain_vehicle/inspection/vehicle_inspection.html