Upselling is when they buy A and you offer them A+. Cross-selling is when they buy A and you offer them B or C.
Getting the customer to you and capturing their attention is the hard part. If a customer comes to you for your primary service or product, when and how do you offer them additional products or services?
The real key to additional business. Ask for the order and sell them something they just didn’t think they needed. Like air conditioning , power steering , power windows and before long they become standard.