You should beware of some potential problems with some affiliates. You should always read the fine print and understand exactly what you are getting into before you commit to one. What exactly do you earn?
Make sure you know you affiliates definition of profit and revenue. Some affiliates pay you a percentage after deducting shipping, credit-card or other payment processing surcharges, and delivery, packing, gift-wrapping and handling charges. Be scared of the ones that pay you a percentage of profits. It is very easy to change the bottom line and before you know if you are earning a percentage of nothing, it's very easy to manipulate the bottom line figures until "profits" melt away like snow in the sunshine.
Be sure to check the landing pages where affiliate links lead to. Some are sneaky enough to include prominent links to other sites and you lose your commission once they leave. Another subtle but effective ploy of affiliate is to include a telephone number for sales. Of course when a order is handled by telephone you earn no commission.