A few thoughts here ,
"Contract" scares away customers, "service agreement" not as much. Make it easy for the call onlys or EOW customers to pay online or with their plastic. A step beyond that would be to require a card on file so that you can charge soon as your complete and not waiting or chasing down coin.
Scheduling/invoice software makes it pretty easy to keep track of non regular work as well. Saves ya some time in the office.
Do your commercial stuff at the beginning of the week as well as the EOW and call ins. I would do one day only of call in work and maybe work some of the call in ppl into getting on a reg schedule by telling them that your full this week.