A nonprofit law firm that charges “below-market flat-rate fees” can’t directly solicit self-represented litigants who are unable to afford an attorney, a Texas bar’s ethics committee said in an August opinion.

The opinion centered on a law firm organized as a 501(c)(3) public charity nonprofit that tries to help “clients who earn too much to qualify for free/pro bono legal services, but also earn too little to afford a traditional private attorney.” The firm can’t directly solicit self-represented, or pro...