According to About.com, job ads that advertise making money from stuffing envelopes are work-at-home scams. These scams typically require potential envelope stuffers to pay a fee for materials or information, so it is very difficult to find a job stuffing envelopes for free.
Envelope-stuffing scams ask interested parties to pay a fee for materials, kits, instructions or simply for the opportunity to stuff envelopes. Such scams either never send the requested materials or refuse to pay envelope stuffers for their work based on vague company standards. They might also offer bonuses for recruiting other envelope stuffers; this pyramid scheme model does not work because there is no money to be made in the first place.