The alert, issued today, says that stamp fraud involves previously used stamps that have not been properly marked as cancelled being prepared and sold for reuse at lower prices than the standard postal service rate.
The commission said that although it did not believe charities were "knowingly profiting" from the collection and sale of used stamps, it was concerned that some were "inadvertently enabling" the fraud by selling packages of used stamps, commonly referred to as kiloware.
Stamp collectors buy bulk kiloware packages of used stamps in the hope of finding unusual or valuable stamps.
But the alert says the "majority" of used, uncancelled stamps are fraudulently resold as if they were valid postage, which makes a "significant" amount of money for criminal gangs and funds further criminal activity.
It warns: "Charities should avoid engaging in this activity unless they are certain that the stamps collected and sold are genuinely being bought by collectors and are not being used for fraudulent purposes."
Fundraisers should be wary of requests from people to purchase UK or GB kiloware in bulk, the alert says.
It says there was no reason for a genuine dealer to request previously used, uncancelled stamps, stamps that did not display prices, or Christmas stamps, in bulk.
Some stamp collectors might genuinely request foreign or specialist stamps, the alert says, but charities should consider anyone offering to deal stamps on their behalf "as a potential red flag".
To protect charities from becoming involved in stamp fraud, fundraisers should "decline requests from individuals or groups who wish to purchase used GB stamps from you directly – either through direct contact or via your online marketplace" and should let Royal Mail know about any suspicious requests, the commission said.
It added that charities should "consider checking that your charity name is not being used in ‘kiloware’ advertising without your permission".