Customising invoices

Is there a way to change the order in which items appear on an invoice? I would like to list items being charged for, then have a subtotal, then on the next line show the holdback which must be deducted from the first subtotal to come up with a second subtotal (which is the amount on which HST will be charged). I have tried to customise an invoice, but I can't get that first subtotal in there. I need to do it this way as it is the standard construction industry format.