Jeremy operates a produce booth at a farmer's market on Saturdays. He buys his produce from a farmer that grows a variety of items including lettuce. Jeremy pays $10 for a box of lettuce which holds an average of 25 heads of lettuce. He sells each head of lettuce for $2. Any lettuce he doesn't sell is donated to the local food rescue organization at the end of the day. History says that this coming Saturday's demand for lettuce falls between 180 and 320 heads of lettuce.

How many boxes of lettuce should Jeremy buy every time (following the optimal order quantity)?