We, like many libraries, use Unique Management Systems  (800) 879-5453.  Unique is a collection agency that ONLY services libraries.  They connect to your automation system so after you have gone through your standard notifications accounts over $25 automatically get turned over to Unique.  You can pull an account at any time.  We pay $9 for each account we turn over to the collection agency and we charge our delinquent customers $10 for the privilege of being turned over.  Our increased return rates on long overdue materials is impressive and if we don't get the materials we get payment for those items more often than not.  You can also turn over accounts where materials have been returned, but fines exceed $25.  You set the levels at each step of the way and pull accounts out of collections at any point.