Use Affiliate Programs to dramatically increase your site's revenue. The internet is new technology, but the rules of real life selling still apply.
People buy on impulse.
But that impulse does not last forever, so you want to complete the sale while they still love your widget. Asking the prospective buyer to mail a check isn't often going to work. Car payment's due soon too. So you have to close the sale while that hunger is still alive. You must accept Visa, MasterCard, American Express and online checks at the least.