Word of Warning.
If you have goods/services that suit these 'Voucher' selling deals via email data bases then I would suggest that they can be a geat way to spend a portion of your advertising budget.
BUT .... its a big big BUT
Some disreputable operators will sign you to deals where you only get your % for redeemed vouchers
This means they simply keep 100% of the funds; which are paid upfront by their punters; who dont follow through by contacting you. That can be a significant proportion.
HOWEVER they know this is; if not illegal (Im led to belief it is illegal); it is grossly immoral. They are selling your product and not paying you for it. No better than white collar theft imo.
You only have to ask and most will change "that clause" to include full transfer of your % of all funds within days or weeks of the deal running.
If you are talking to these dudes make sure you know what they are offering.
Anyone looking at any more than 30% withholding % just isnt in the market. You can get way less than that if you deal. Contact some others.

And ask for an 'audited' circulation of the area you are marketing to.

Its not just the sharks in the water you need to watch out for [}:)]