olemus
ühe müügimeetodi siht: panna klient oma ostule veel midagi juurde ostma (tarvikuid, uuendusi, teenuseid vms)
Wiktionary:
to persuade a customer to buy more items, or more expensive items, than they had intended

ülevaateid
https://en.wikipedia.org/wiki/Upselling
https://blog.hubspot.com/service/upselling
https://www.bigcommerce.com/ecommerce-answers/what-difference-between-upselling-and-cross-selling/
https://optinmonster.com/upselling-tips-and-examples/