Get a Fast and Reliable Web Site
Most people have a very short attention span on the Web. People have no time for sites that are slow to load or hard to understand. You need to concentrate on these aspects of your site and forget all the flashy graphics or award winning designs. E-Commerce sites need to be functional first and foremost. (If you want to see the best designed site on the Web look at Google.com)
The worst sin an E-Commerce site can commit however, is to hang or crash out in the middle of a transaction. Customers are left unsure whether their order has been placed or their credit card has been debited. Believe me, they don't like this and will avoid your site like the plague if they have this sort of experience.
Nothing gives a customer a greater feeling of confidence than a confirmation email appearing in the inbox seconds after the order is placed. It's easy to automate this, so there is really no excuse for sending out an email two or three days later saying, "By the way we got your order". An email confirming dispatch or delivery of the service is also a "must do".
Process Credit Cards On-Line
Credit cards can be processed on-line or offline, but why not do it properly. You shouldn't really expect your customers to wait for several hours (or days) before informing them that there has been a problem with their payment. Customers like to be told when they place the order that everything is OK and the order has been processed. It might be more expensive to set up real time processing but it's more secure (you won't be storing credit card details on your site) and your customers will appreciate it.
Ideally delivery should be immediate (for downloadable software and subscriptions) or next day for everything else. Obviously in some cases this will not be physically possible, or will be prohibitively expensive., however it is an ideal worth aiming for. The shorter the time between the customer placing the order and delivery being completed, the more satisfied the customer will be, and more likely to repeat the experience. (Even if next day delivery is expensive it's worth making the option available, at extra cost, for these customers that just can't wait.)
E-Commerce has a credibility problem in that while it sounds great in theory, it is often slow and cumbersome in practice. By concentrating on satisfying the customer as quickly as possible you will give your business a huge advantage over your competitors.