Which of the following is NOT a type of document served by private process servers?

Private process servers are primarily tasked with the delivery of legal documents necessary for the judicial process. The correct answer, checks for payment, is not a type of document typically served by private process servers because it does not relate to legal proceedings or disputes that require formal notification or service of process.

Eviction notices, subpoenas, and complaints are all legal documents that require formal service to ensure that the parties involved are aware of legal actions being taken against them or need to fulfill certain responsibilities, such as appearing in court. Checks for payment, on the other hand, do not fall into this category; they are generally financial transactions and not part of the legal documentation processes that private process servers are engaged with. Therefore, checks for payment are excluded from the types of documents that private process servers handle.