Per the Buying Comics help guide.
You may cancel an order, or items off of an order yourself, any time before it ships, and receive a refund. To do so, log in to Atomic Avenue, and click on the order number. In the order window, click the Cancel Order button. (To cancel only certain comics, mark the checkbox next to each you want to cancel, and then click the Cancel Selected Issues button.)
I have had two different customer that wanted to change their order before I shipped them but they could not do what the help says. So they e-mailed support and you had them e-mail me to do the cancelling.
That gave me negative points since I did the cancelling. If buyer can cancel, maybe a step by step with pictures will help them. If the cancelling is only before submitting the order, then you need to update the Help and try to get that function in the future.