Most affiliate programs (commission-based ones) pay a certain percentage of the revenue your site visitors generate. But the meaning of the word "revenue" can vary depending on which site you're looking at.

Many programs pay commissions on the gross value of a sale; that is, the amount of money that the site will actually receive in payment for the goods or services being ordered. This always excludes additional charges for credit card transactions, and delivery charges.

A few sites apply much less generous definitions of "revenue", for example only paying commission on the profit of a sale. It's very easy to manipulate the figures until the visible profit simply melts away. You're likely to earn a very small commission if you sign up for this kind of service, as the profit margin will at best be about a quarter of the total value of any sale, and your commission will be a small fraction of this much smaller pie.

This issue of "revenue" is often buried deep in the Affiliate Agreement you'll have to sign when joining the program. So read the Agreement carefully and make sure where you stand before you get going.

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Watch Out For Returns!

Be aware that virtually all affiliate programs only pay you for sales that have actually completed and where the customer has opted to keep the merchandise. If a customer returns the goods for a refund, your commission will be cancelled; you may even be sent a bill for the amount of the commission if you don't have any earned credit outstanding with the company in question. This doesn't happen too often, but be aware that the possibility exists.

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What Is Commissionable?

Be clear exactly what you'll earn commission on, and how that commission will be calculated. For instance, some affiliate programs will only pay you commission on the specific item you link to, and even then - only if the visitor fills in the payment details immediately without clicking on any other link out of the order page (a very unlikely scenario). In this case, even if your visitor goes on to buy 30 different candles from Candles R Us, you would only see your commission if they bought the Orange Citrus-scented Candle that your site's link pointed to.  Most programs do pay commission on all the items sold during a single visit to the site, or even in some generous cases on all the items sold during a given period of time from the date of the visit (up to 30 day's after, for instance)  - just be sure to read the Agreement.

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A Quick Look at Cookies

Many sites make use of a tracking technology known as "cookies" to keep track of a visitor's progress across the site. A "cookie" is a small piece of data that is stored automatically by your web browser. Inside the cookie, the site will have encoded a certain amount of information about the customer, such as a unique tracking code or address details.

The problem with this approach is that cookies can be refused, discarded and eaten. Some paranoid web surfers, worried about their online privacy, systematically refuse to accept cookies from web sites they visit. In this case, their purchases will not reward you in any way since the site has no way to track them. Cookies will also be discarded when the storage area for cookies fills up (depending on your web browser, the storage area has space for up to a few hundred cookies). If you're participating in a program where you are supposed to receive income over an extended period of time, you may well find yourself empty handed if the cookie expires and gets overwritten by a new one. Finally, certain system maintenance programs also "clean" your computer by deleting cookies.

Unfortunately there is very little you can do about this problem, except be aware that it exists, and that it may have an impact on your results.

Where's My Check?

You need to know how long it will take for your check to get to you. Some affiliate programs pay fairly promptly, a few days after the end of each calendar month for earnings during the previous calendar month. Other programs pay quarterly, while others may pay twice a month.  It all depends on the setup.  If you sign up with a program that, for instance, pays 90 days after the end of each calendar quarter - be cautious!. On the Internet, anything can happen in a few months: bankruptcies, server crashes and glitches etc. - so be wary of a program that will hold onto your earnings for months on end.
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