It requires a lot of investment to get a new client in any business. Client acquisition of new clients has probably never been harder and advertising costs are likely to cripple some businesses (don't let that be you or your clients).
Counteract this expense by selling more to your existing customers. All you need to do to be able to sell more to existing clients is to nurture them before, during, and after the initial sale. It cost far less to add more tiers, products & services and sell these to your existing clients than it does to find new customers because, with an existing customer, you have already incurred the initial acquisition cost.
This cost can be covered in most cases with additional upsells at the initial point of the original sale.
When you have the right systems in place to continually nurture your existing customers, you can not only make upsells at the point of the initial sale but also by following up and making future sales.
You have done the hard work initially to win over your customer's trust and faith in your company, provided you continue to serve them well a good percentage of customers periodically will continue to buy from you over many years.
We call this the lifetime value of the customer or LTV. A backend sale is an easy sale to make because you have won your client's confidence. So selling them more additional products provided they precede or complement the initial sale is just providing excellent customer service.
Let's imagine a customer buys a camera it would be very bad service not to remind the customer they might need spare batteries, memory cards, lenses, tripod, and bags and that's just at the point of sale.
Imagine over time you build a relationship with this customer you might offer them training, webinars, books, courses, and much more.
All these products and services are possibilities even if you don't stock the product yourself.
You could either sell another company's product as an affiliate or alternatively you might use these additional products as part of an upgrade bundle add-on service.