One of the biggest problems that e-commerce stores have to deal with, when it comes to not making the sales that they want to, is abandoned shopping carts. Consumers who go all the way through to shipping and then decide not to make the purchase may seem like a mystery to the store owner, but there are actually a number of fairly common reasons that consumers do not complete their purchases. If you deal with these problems within your own store, you may find that you get a lot less abandoned carts.
Although you obviously don't want to lose money on shipping, when the price of the item or items the consumer is buying suddenly shoots up due to shipping, they are more likely to head elsewhere to see if they can get the goods at a better price. If you can save money on shipping in any way then it is more than worthwhile.
Another common reason for consumers not completing their purchase is if they realise that their order is going to take weeks to arrive. People shop online for a quick and convenient experience, and waiting more than a week for an item is very likely to put a consumer off, sending them elsewhere.
A consumer wants to be able pay for an item easily, without having to create an account anywhere for find a card which will work on the site. Make sure you have plenty of payment options listed, including debit/credit cards but also including something like PayPal, which many people prefer as a safer way of paying for things online.
The longer it takes to checkout and the more steps a consumer has to go through before they can pay, the less likely it is that your consumer will complete their payment. If you can allow consumers to make a payment without having to sign up, this will be much better for you and can save hundreds of shopping carts each year.