When printing a client statement as an invoice, Therabill formats that remit stub to fit within 3.5 inches from the top of the paper. Therabill also places the detach line at 3.5 inches from the top of the paper. If you are wanting to print to perforated paper, then you will want to order perforated paper with the perforation at 3.5 inches from the top.
The perforation is not lining up with the dashed line (the detach line).
Therabill creates the PDF with the dashed line 3.5 inches from the top of the page. When printing, make sure your printer is not introducing any scaling (for example, fit to page) or margins. If you printer is introducing margins or is scaling the dimensions, then the detach line will not line up with the perforation.