Alan,
You can try calling TCM service booking center, look for Pamela Koh. Tell her your ride rear suspension is too soft, with three passengers, everytime hit the hump on the MSCP. And you also learnt about TCM provided "renew rear coil spring" for those with the problems.
You can also look for Stephen Ang at Bukit Timah service center. If none of them want to help you, you then ask for their manager. From the forum posting, there are already few bro changed their coil spring with TCM, so why not for you?
Good luck!