1. No persistent cookie
As I wrote previously, persistent cookies that remember what's in a customer's cart are important for customers that take more than one visit to complete a purchase. Without persisted carts, those who abandon the checkout process with the intention of returning later must start all over again.
2. Required registration / account sign in
According to Forrester Research, 23% of shoppers will jump ship when asked to register with your site. If that's not bad enough, even your registered users lose heart when they can't remember login information. “Forgot password?” links are not the answer. Analyzing the database of a very large online retailer, Jared Spool found only 25% ever tried to finish the checkout after requesting password information! Safest bet is to offer guest checkout with the option to save customer information at the end.
3. Trapping customers in your checkout
Many conversion gurus have preached the virtues of removing site navigation during checkout to minimize distractions and keep customers focused on the big shiny calls to action in the checkout process. But this makes it difficult for customers who want to go back and add items to their order (which increases average order value). For example, I recently made a cross-border purchase. The shipping charges to Canada were so high, I wanted to add to my order to justify the shipping cost, but was unable to do so without opening a new tab and visiting the site's home page again.
While I'm all for removing clutter, and game for every site testing with and without navigation, I don't think it's necessary to remove all means for the customer to back out of checkout. At the very least, hyperlink the site logo to the home page.
4. Surprise shipping costs
Speaking of high shipping charges, this is the number one reason for cart abandonment according to Forrester Research. While how much you must charge for shipping is a business issue, not a usability issue, showing shipping charges too late in the process was also a customer peeve. 59% of consumers expect you to offer a pre-checkout tax and shipping calculator on the shopping cart page.
5. Asking for too much information
12% of consumers abandon checkouts that ask for too much information (again, thank you Forrester). A classic example of asking too much is Foot Locker, which makes favorite sports and shoe size required fields. Huh?
Take a look at your checkout forms and ask yourself which fields are untouchable and which fields you could sacrifice for a split test.
PS - stay away from captchas too. Only 1% of checkouts use them, they're difficult for customers to decipher, and don't put it past spammers to find workarounds for them.
6. Poor error handling
Forms that contain input errors are culprits to cart abandonment because customers need to understand what to correct. You must make the error message very easy to spot, very easy to understand and very clear why the error happened. Many checkouts fall flat in this area. Here are some examples from top e-tailers:
American Apparel packs so much above the fold that you can't see where the error occurred when the page refreshes. If you're lucky, you might notice the yellow strip containing the error message.
Chapters Indigo highlights all the errors at the top, which forces the customer to figure out which fields are associated with each error.
Worse, Skull Skates uses a dialog box that must be closed before you can continue. You can't bank on customers to recall each error if there are more than one.
Williams Sonoma and American Eagle Outfitters do a better job, respectively.
Beware of browsers that can mess up the display of errors, making them near impossible to read.
Industry jargon like "AVS mismatch" should also be explained. Most consumers don't know that the address verification service requires an exact billing address match to their credit card statement, including middle initial if applicable. This is especially problematic when the Shipping Address can be copied to the Billing Address section with one click and the online retailer uses AVS.
And don't forget login screens, a common place for errors.
7. Not explaining the CVV
Like AVS, CVV (or CVC) is an ambiguous acronym. Make sure you explain it.
8. Not optimizing for page load speed in checkout
Do you focus your performance optimization efforts only on your home page or pages? Don't forget to optimize your checkout, both for page load speed and your payment gateway performance.
9. Pre-checking fields
While it won't impact conversion, it can erode trust post-checkout when customers unwittingly opt in to your marketing program and receive what is in their minds unsolicited email (aka spam).
10. No contact information
If a customer is experiencing a problem in your checkout, a prominent customer service contact number (or live chat link) may save the sale. So why do just over half of retailers (according to Jupiter Research) display one in checkout?
There are many other checkout nightmares that could happen. These are my top 10. I welcome your ideas in the comments.