You need to have a seamless system in place when working with small businesses especially when billing clients. I found the BEST invoicing software! I've used a ton of different programs for my mobile web business and then came across this FREE gem. It's called Invoice Bubble and it's a web based invoicing system.
(Not an affiliate link, am just a big fan.)
Just sign up for a free account and play around with the software. It integrates nicely with Paypal. If you have any questions please let me know.
PS: Oh and don't forget to protect yourself. Use my small business contract or come up with your own because not getting paid sucks!