The next site only charges £5 for postage, so I sign up for another account, find my credit card and fill in the details. But wait; this site didn't tell me in advance that they don't show the VAT in the price, so it's no saving after all. I abandon the cart halfway through the credit card validation; their card processor might or might not charge for that. Third site, another account. Postage is £5, there's no hidden VAT. I type in my credit card details again. They're not accepted, because the default is Visa Electron, not Visa and it was pre-selected, making it easy for me to forget to change it. They got the sale, but they burned more cycles on it than necessary; they may have done the initial card type/number algorithm check locally but they still had to serve another page. Do that to a thousand people a day and your CPU and hard drive and network and aircon are all working more than they'd have to if you designed things properly in the first place. And your customers are less satisfied and less likely to come back.
Not showing the postage cost before checkout is the number one reason for people abandoning a shopping cart online (in the real world I think it's because it's too much effort to push it back to the shop unless it has one of your shiny pound coins in). If you don't show me exactly how much you're going to charge, I'm not committed to buying; until I see the final price, I'm still browsing and your site is wasting my time.
A lot of people abandon sites that collect too much information up front. Why do you need an account for me; I might never come back and now you have my personal data clogging up your hard drive, imposing a duty of care on you to back it up and protect it from hackers. You can't spam me or sell me to an address list; you didn't ask for permission or I said no. Offer me the chance to save my details after you've delighted me with a friction-free shopping experience. Or if you insist on my address up front, use it to work out the shipping charges and show them to me.
Ludicrous postage and packing charges. I paid £8.50 postage on an eBay item, special delivery. The stamps on the parcel didn't come to £2.50 let alone £8.50. I've been charges $20 shipping on an $1 eBay auction. £8.50 for something that weighs 100g? Please. For that price I could get a travelcard to Tottenham Court Road, shop with one of your competitors, buy a latte and still have change. How much will I pay for convenience? Amazon has doubled the number of people who are willing to pay $79 a year not to pay any postage at all.
Is it in the basket? Is it really in the basket? Is it still in the basket? I had to go answer the phone. Your shopping cart isn't showing me what products I put in and I forgot. Give me a hand, show me what I'm doing.
Ecommerce; it's not rocket science but if you get it wrong, your sale goes bang.