Ok, so I think I've solved that problem for the most part...I removed the exit() and just returned the output, is that correct? I now have my text, but in front of it is a bunch of text I don't know where it came from...haha. Mystery text. I get this:

Order complete
You are currently viewing the possible transitions to and from workflow states. The state is shown in the left column; the state to be moved to is to the right. For each transition, check the box next to the role(s) that may initiate the transition. For example, if only the hulking_editor role may move a node from Review state to the Published state, check the box next to hulking_editor. The author role is built in and refers to the user who authored the node.
An error has occurred during payment. View the response below and either re-try your order with another method or contact us for help.Transaction Response Code: 1Transaction Response Code Description: Unknown ErrorMessage: Unspecified Failure

The order complete is the heading, that's fine. The bottom text is my text, which is correct for what I entered for the order...but the first paragraph has just appeared. Do you think that has something to do with the page I've returned? Anyway, I've attached my payment gateway, I know you can't do much with it, but maybe you will notice something I've done wrong?