For people who are paid under the table and may not have clear wage documentation, is there an option for a self-declaration of their income?

It would depend on a case-by-case basis. The easiest would be to qualify under one of the other criteria. If, for instance, the participant is enrolled in one of the federal programs (i.e. SNAP or Medicaid), the recommendation would be to qualify using that. Another option would be if someone else in the household qualifies (i.e. a student who attends a CEP school), that would also be an option.