Under Washington law, you are entitled to minimum wage plus your tips. However, there is no rule against pooling tips, and no rule against the owner or supervisors being a part of the pool. In essence, your boss has changed the employment contract. Where you have no union, and are not a civil service employee, then unless you have a very unusual employee policy manual that would vest you in the prior tip policy, you have no recourse but to change jobs.
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Why are two people going on each delivery? Surely your boss has better things to do? Tips in many different kinds of businesses are often pooled, so it may be ok here also.
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