When going through the checkout process, customers in the process of paying for items are bound to make a few mistakes here and there.
The key here is to handle these problems smoothly, enabling customers to correct any errors they make without causing unnecessary frustration.
If they have to do too much work to figure out why an error was made, and what they have to do to fix it, they are likely to give up on the whole process, and you will have lost a sale.
The key here is to make the process as simple as possible to avoid common errors, and to make them easy to fix if they do happen.
Here are a few suggestions...
Make the process easy to understand
However well designed your checkout process is, human nature means that customers will make mistakes. Making it easy to use will minimise the number of errors though.
Adhere to web standards and conventions
Customers will expect your checkout forms to be broadly similar to those they have used on other e-commerce sites, so any departures from convention are risky.
For instance, users will expect an asterisk or shading to denote a form field which must be completed, so any deviation from this is likely to cause confusion in the customer's mind.
Flexible form input
Don't be too insistent on entries being in upper or lower case, as this just makes customers have to work harder to figure it all out. You should allow customers to enter postcodes in lower or upper case without a space, for example.
Make error messages clear
When people have made an error, let them know exactly what they need to do to correct it and get back onto the purchase.
I got this message after entering credit card details at Oasis, which was far too vague to help me correct the problem. It didn't specify where the problem was in the form, meaning I had to check every single part of the form.
After double checking and still getting the message, I ended up giving up on the purchase, something many other customers would have done in the same circumstances.
Comet does a better job here, after I forgot to enter an evening phone number, and made a mistake confirming my email address:
This time I am told exactly the mistakes I have made when completing the form and can easily figure out how to fix the problem.
Highlight the field(s) containing the error
This is another way to help the customer recognise where a mistake had been made and correct it. Highlighting the form field by shading or placing red error text next to it will help avoid frustration.
Don't delete information already entered
I have encountered a few websites that will, in the case of an error made in one form field, will automatically delete the information in the whole form. This is guaranteed to annoy customers.
Make error messages polite
Even if the user has made an obvious mistake, the tone of your error message should be friendly and polite. Pejorative terms like illegal or invalid should be avoided when referring to errors, while capitals are also not advisable.
Basically, the message should not blame the user, but seek to help the correct their mistake.