No, you don't. That said, we cannot emphasize enough that you should not do this on your own! Our TPA Partner, BASE Administrators, has invested substantial resources to keep it compliant with tax, benefit, labor, ERISA and other laws. They provide year-round customer support, a process to set-up and maintain your plan document.
By having a TPA, you also have an outside professional that reviews your claims to make sure that they are allowed and properly categorized. This third-party review adds to the credibility of the documentation and substantiation.