Is there a time restriction where we can go after a past-due invoice? Using a collection agency did not help so I was thinking about taking legal action.
Yes, for many legal claims, there are time restrictions on how long you can wait before asserting a legal claim. This is referred to as a "statute of limitation", and the length varies depending on what kind of claim that you want to bring. It is very important to commence any legal action within the limitation period to avoid losing your legal rights. More detail about your situation is needed to say with certainty, but one period of limitation to consider in your situation might be Pennsylvania's 4-year period of limitation for contract claims. You may have other claims that need to be brought in less than 4 years. I recommend contacting an attorney for help determining whether you still have a legal claim, and how to enforce it.