This is a form of advertising by which the website receives a small commission if the reader clicks on a hyperlink, goes to the online store in question (e.g. REI, Backcountry, Sierra Trading Post, etc.) and subsequently makes a purchase.
These fees are paid by the online retailers, not by readers, and in no way affect the purchase price of the said product/s.
The proceeds from these commissions help to support The Hiking Life website (i.e. maintenance costs and development of new content).