At present it is not obvious to clients that they have clicked the button to process an order. Oh sure the browser's load page icon starts swirling but that is small and many people don't pay attention.
I have noticed some orders can take ten to fifteen seconds to process. This means clients need to wait but since the screen doesn't change in anyway people get impatient. Therefore I suggest the following change to reduce ordering errors and help with impatient buyers.
1) After client has filled out all order fields like name, email, phone, selected payment method, etc they click the button to place the order; as they currently do.
2) WHMCS checks to see if any required fields are missing and if so warns the buyer; as the system does now.
3) If no errors exist the page changes to a simple message saying something like "Please wait while we process your order...". There should be an animated gif like arrows spinning, a ball bouncing, a bar moving, etc. This is the new part.
4) Once the system has processed the order the results, either success or failure, are shown to the buyer; as they system does now.
The only change from the current method is that please wait message. The whole page should change to make it very clear to the client something is happening instead of the current process where it looks like nothing is going on.