I also use PayPal pretty much exclusively, although I do accept checks -- just never have had any client pay me by check.
One thing I found with PayPal's invoicing is that it isn't very personal, and for me, not detailed enough. I created an Excel workbook, which serves as a time tracker as well, and I like to send this out prior to invoicing with PayPal.
I'm not sure you'll find it useful, but it will calculate your total time, etc. I think it's pretty cool

. Please feel free to check it out and use it, comment on it's good/bad points. I'm always up for improvement!
I tried to upload my workbook, but it looks as though Excel is not a valid format

. Is there a way around this? I'd love to share!