The most profitable sale
In any business there is one type of sale that brings the most profit. There is one single sales activity that can make the most difference in the profitability of your company. Not taking advantage of this all-important sales opportunity will cost you thousands of dollars.
Source: Sales trainer/coach Howard Partridge
That sale is called the "upsell" or "add-on" sale. The reason this type of sale is so profitable is because you have already invested the cost of acquiring and administering your client.
Therefore, anything that is added to the "ticket" is extremely profitable. The marketing cost, the labor cost, the administration cost (i.e., the overhead), has already been realized. This makes the add-on sale the most profitable sale you can make.