Well, a lens is not like a ball head or softbox or light tent. It is expensive, fragile and a lot of concerns in condition like scratches, de-centered elements, focusing issues. Buying and transporting and storing lenses requires a lot of care and effort just to make sure. So if you ask some one to buy and bring back, will you indemnify him/her from all responsibility? If not, who is responsible for what? If nothing happens and you get a perfect copy, all is nice and great. But what if there are problems? Warranty will not cover everything. What if the person go to the shop you pointed out and the price is different? What if the lens is shipped to the wrong address and cannot make it in time for the flight? What if the person get stopped at customs and is asked to pay gst? What if there is sales taxes at the location of purchase( sales tax is now as high as 10% in some cities)? Always ask the what ifs.
It is not just about if someone will disappear with the money. So much can go wrong from lens selection, to pricing variation, to technical problems of the lens, to transportation risks, to problems in lens deliveries to hotels or wrong addresses, to wrong item shipped, etc etc.
When people here tell you is hard for strangers to do it for you, it is not because they are not kind or they are out to make life difficult for you. But most have done it before and have gone through the problems. If you think it is a simple help-me-buy-bring-back-all-is-happy, you are being too optimistic and over simplifying the whole thing.
To neech7, it is gracious of you to offer to be the mule for TS. I strongly suggest that you not handle any money. Money is always very sensitive. Better to let TS do an online credit card purchase and put your address as the shipping address. All you do is to bring the package back to SG. That is the best approach for all overseas purchases. I do that even with extremely good friends or family members overseas.
Anyway, good luck.