Most of my clients I bill by the hour but one I bill by the page. Is there a way to create an invoice that lists the number of pages and per page rate where usually the time and hourly rate would go? Without deleting the data I collect on my time?

If you've found a way to do this, I'd love to hear it. I am able to bill by the quantity, but I can't seem to make it appear that way on the invoice for some slips, but not others. I use quantities for some things, but others are just flat rates without time or quantity. Wondering how to reflect this on the invoice.

For this I'd use the quantity slip.

You should be able to show the option to add the number/qty in the Invoice options.

Send invoice -> Preview -> Option button.