Recovering money owed - IL customer, MA supplier

My company acted as a supplier of services for an IL customer...they are refusing to pay the last outstanding invoice ($4.5K) and dodging the calls/requests/etc.

Any efficient way to get their attention and/or try to recover this money owed? We do not care about the relationship, so a lawsuit appears to be in order, but any thoughts on how to make it cost efficient vis-a-vis lawyer fees/etc.?

They are in Cook County (Chicago).

They have a web presence and customer-facing 'reputation' so maybe even the threat or filing of some paperwork would be a wake-up call?

Thanks!