Whenever I needed someone to respond promptly to my business emails I would clearly mark them urgent, your attention please, in whopping bold capital letters in the subject line and anywhere else applicable. If I then heard nothing from them, would cancel their order and confirm cancellation with one last email requesting they contact me again should they wish to re-order. Job done, no drama and move on - they really have no cause for complaint as you have been a good communicator, been polite and you have left the door open for further transactions.
Yes, customers are gold and should be treated with respect, great service and quality products. By the same token you are running a business, you cannot be wasting time/product/resources trying to second guess someone who couldn't be bothered communicating with you. It is not a matter of being hard or unkind you need your business to survive.