Given multiple products, this cost is distributed over a number of sales, and thus is less per sale. More important, an existing customer is your best prospect for a subsequent sale. While there are exceptions, such as a successful affiliate program marketing say a private site, a single product site will produce modest profits. While they may be sufficient for one building a hobby or personal site, they are unlikely to produce a livable income. If you have a single product site, or are planning one, the path to increased profits lies in adding additional products.
Customer loyalty is the goal.
You want all to return for more. Given shopping bots and so many web shoppers determinedly looking for price only, building repeat business can be difficult. Still, even though many buy never to return, your best prospect remains previous customers. To take advantage of this, you need additional products of interest to them. As you consider adding products, be clear about your target.