I know there are a lot of people out there that charge flat monthly rates for thier service. A lot of people I talk to that bill in this manner ask how to guarantee payment and avoid chasing people each month for their money. Something we did years back was create a "Payment Booklet" just like you get when you buy a car. It worked wonders. Once we agreed to a set monthly rate that did not change unless there were add-ons (which were then billed separately), we had our local printer make payment booklets with stubs. Their amount, date address and our return address were all located on the stub. Their due date was also listed and they always knew when to pay. We avoided so much time in billing and dreaded chasing. We lined up our clients in groups - some payed between the 1st-10th, others 10th-20th and others 20th-30th. We made it clear in the contract that there would be a penalty for late payment as well, which no one ever questioned. Our clients truly appreciated the convenience, and some even were ambitious (or generous) enough to pay several payments in advance so they did not have to worry about it during vacation months, and so on. Just thought I'd throw this out there. Hope it helps.