A taboo (or no-go) is a practice that a sex worker never offers—regardless of whether an extra fee is offered. These are often explicitly listed on the profile or in the service catalog (e.g., "Taboos: anal, golden showers, active golden showers").
Taboos are non-negotiable. Attempts to negotiate or pressure a sex worker regarding them are unprofessional and, in the reputable industry, grounds for immediately ending the appointment—with full payment for the time already spent.