Bill collectors often call every day. Simply sending a PayPal Reminder and or an email reminder with invoice is easy. I've done it. It works. Sending by email is different then taking the time to send a printed invoice by mail. Much easier and actually more effective IMHO. I believe once a day is perfectly legal. If they feel you are harassing them they must tell you though in order for them to take you to court. They aren't likely to spend money to take you to court if they aren't willing to spend money to pay the bill. Often one owing money is NOT likely to take that step. I've had to collect and regular emails are more effective than going to small claims court. You'd likely need some means to collect if you won (check with bank account with money in it for example). Winning in small claims often doesn't give you a much better chance at collecting. There's a good reason why bill collectors resort to regular phone calls over court.

Your objective is to collect the money, not go to court.

I was once hired as a freelancer by a large post production house, editing for a syndicated TV series. They owed me 3 months pay after I completed the job. After six months they still had not paid despite my calling a couple of times of weeks after a "reasonable" six week period. I called them and said I would be coming down to pick up my check and gave them a specific time. When I walked in, the receptionist had the check.

Not quite picketing but it worked. 25 years of experience and YES sending emails work and YES going down to their "office" works (you'd have to leave if they asked but you can certainly state your purpose in "stage" voice for those present to hear and still not shout). As long as you don't threaten them or call a score of times during the day you can apply a reasonable amount of pressure to collect.