Final answer:
The legislative act that provides requirements for written payment plans, like the one for cosmetic surgery, is the Truth in Lending Act, which ensures disclosure of credit terms.
Step-by-step explanation:
A patient who agrees to and signs a written payment plan for cosmetic surgery costs, paid in installments over a period, is protected by the Truth in Lending Act (TILA). The TILA requires creditors to disclose credit terms to consumers in a clear and meaningful way, to facilitate the informed use of credit, including details about payment schedules and the cost of finance charges. This act is particularly important in situations where healthcare, such as cosmetic procedures that insurance may not cover, requires financing, and the costs can be high.