Should I use a collection agency instead of suing over an unpaid invoice?

This is genuinely worth considering as an alternative path. Collection agencies typically take somewhere between 25% and 50% of whatever amount they successfully recover on your behalf, and they generally charge nothing upfront to attempt collection. On a $22,000 invoice, for example, that fee structure would leave you keeping somewhere between $11,000 and $16,500 of the total, depending on the agency's specific rate.

Filing suit yourself keeps the full recovered amount for you rather than splitting it with an agency, but it also means you personally carry the ongoing work and the risk that the case doesn't succeed. For debts that are quite old already, or where the debtor has become genuinely difficult to locate or trace, a collection agency's existing resources and infrastructure often make it the more practical option.