Whether or not your employer can claim the tip credit depends on the amount of time you spend on balance, in tipped versus non-tipped duties and you make at least $30 per month in tips. Most waitresses and waiters perform at least some duties other than serving customers. If you are a prep cook who runs food out to one or two tables during a busy period, that is very different. Flat rates are discouraged but not illegal so long as you qualify for the tip credit, the number of hours worked divided by the rate equals the federal minimum of $2.13 per hour, and you make at least MW when tips are added.
Example: You work 5 hours and get paid a flat $25. You make $30 in tips. The $5 an hour your employer is paying over the $2.13 an hour required so they are clear on that point. If you make $30 over the 5 hours, that works out to $6 an hour, added to the $5 from the flat rate, means you are well over MW.