Both cross selling and up-selling involve a customer buying more stuff from you than they’d originally intended. But they’re very different.
Cross selling versus up-selling
cross selling involves selling related goods or services. If you’re in the process of selling someone a wedding outfit you could also offer them a matching hat, shoes and handbag
up selling involves selling a more expensive or exclusive version of the same thing. If you’re in the process of selling a £99 tent you could persuade your customer to buy the next model up for £120