The Beach Dude $($BD$)$ employs a legion of current and former surfers as salespeople who push its surfing$-$oriented products to various customers $($usually retail outlets$).$ This case describes BD$$s sales and collection process.

Each BD salesperson works with a specific group of customers throughout the year. In fact, they often surf with their customers to try out the latest surf gear. The BD salespeople act laid$-$back, but they work hard for their sales. Each sale often involves hours of surfing with their customers while the customers sample all the latest surf wear. Because BD makes the best surfing products, the customers look forward to the visits from the BD salespeople. And they often buy a lot of gear. Each sale is identified by a unique invoice number and usually involves many different products. Customers pay for each sale in full within $30$ days, but they can combine payments for multiple sales.

BD manages its clothing inventory by item $($e$.$g$.,$ XL BD surfer logo T$-$shirts$),$ identified by product number, but it also classifies the items by clothing line $($the lines are differentiated by price points as well as the intended use of the clothing, e$.$g$.,$ surfing products, casual wear, etc.$).$

Draw a UML class diagram that describes the Beach Dude$$s sales and collection process. Then, use your diagram to answer the following questions about this diagram.