Answer:
Kevin has analyzed the situation well. However, he should also consider the fact that he saved $10 by only purchasing the shirt.
Opportunity cost is the cost of the forgone alternative. Out of the 3 choices, he only purchased 1 of the choices, the opportunity cost are the other two choices. However, he is still capable of buying the flip-flops costing $10 but he chose not to do so. He should consider it as a savings aside from it being a lost opportunity.