Is it Illegal to Put Flyer distribution on letters or doors in Singapore?

The answer is no. Door to door flyer distribution is not illegal and the town council actually provides a box for every household, in which the flyers are collected and contained. 
 
 
When Does Flyer Distribution Become Illegal?
 
 
Flyer distribution in Singapore is mostly a legal way of advertising your product or services. But there are times when it becomes illegal. Flyer distribution, remains legal, unless the distributors enter a private property, which can be considered as trespassing. Nevertheless, since there are already flyers boxes provided for the household, it will be almost impossible for flyer distributors to trespass, all they have to do is to put the flyers into the designated bins. Flyer distribution also becomes illegal if the flyers are political in nature, which requires certain permits. 
 
 
If flyers are just for commercial purposes – promoting and advertising products and services – distributing them on the streets is legal. 
 
 
Flyer distribution is legal when done in public property as long as it is not stated otherwise. Distributing them on private properties, such as shopping malls, SMRT, and other private establishments depends solely on the management of the establishment in question. 
 
 
The entities’ control of the establishment, however, can be gauged by its smoking areas. For instance, The bus station or train station is a non-smoking area and you will see smokers at one end. This area is where the public property begins and where you can start handing out flyers. Sheltered areas or areas near the top-up counters are considered as part of the station and most likely flyer distribution is not allowed in there. 
 
 
Shopping malls, on the other hand, only allow flyer distributions if they are conducted by their tenants. If you are NOT a tenant, you can probably make arrangements with the mall’s management for a fee.