I don't know about printing it on the bottom of the page. You can go to Tools>Options>Global Billing and use #7, the Standard paragraph to print whatever you might want on the Invoice, but it only prints it at the top of the page, under the Date.

I think the right to charge interest on late payments should be covered in your engagement letter first, so that clients are put on notice.

Why does setting the global standard paragraph on the Global options not carry to every invoice without checking the box  that says "Caution:  Check this box to remove every field and the the global options will flow to the form....." at the top of each invoice?  What does "Global" mean in the tools?    And it does remove any flat fee bill amount you had put in.

Why is Standard Paragraph # 7 in the Global Billing Options and Paragraph # 8 in the invoice, which shouldn't matter?   It does flow when you have to waste time with every invoice with an extra box to invoke what are supposed to be a GLOBAL BILLING OPTION.

Why are other items global but not the standard paragraph?

Not only that but if you use the box it really does remove things, i.e. the billed amount that was put in originally, so you have to use the dubious, duplicative, redundant box before you put in the amount.  

Why doesn't it remove the flat fee description nor the discount description?

What is the reasoning behind this alleged "Global Billing Option"?  What are you thinking?