The Nature Society used to raise money through a Stamp Fund where they would collect old stamps and sell them in bulk to stamp dealers. However, given the lack of information about this on their website, we believe they have discontinued this programme.
Postage Stamps (Used)
Postage, Postal, Old
Recyclable?
Blue Bins & Chutes
Other Recycling
Recommended
Recycle as Paper
Do not remove stamps from envelopes unless you plan to give them away. Small bits of loose paper get filtered out of the recycling system.
Recycle as you would any paper. More info here.
Alternative
Buy or Sell
Have you just inherited a stamp collection or find your stamp collection no longer sparks joy?! For those with large collections or special stamps, there are people online buying and selling.
While a lot of this trade is done through shops (philatelic stores, coin and stamp stores) there are some some sales done through Carousell.
You can also join events organised by the Singapore Stamp Club. It has regular member gatherings and stamp fairs open to the public. Follow them on Facebook.