In general, Profit is the difference of Total Sales and Total Expences.
In formula form we could write it,
Profit = Total Sales - Total Expences.
According to given problem, sales increase by $1500 in a week.
That is some more sales also added to previous sales but still some expences are included in total sales and even in increased sale $1500.
So, in order to get the net profit, we need to subtract all expences (even the original cost in making the product and expences of all servises provided in sales).
Therefore, we could finally say, "if sales increase by $1500 in a week, profit would not increase by about $1500".