Anything you buy at best buy, radio shack, any appliance, computer, printer, cellphone, & tv you buy, they will try to sell you a warranty at your local big box establishment. It must be work, as everyone is offering it for consumer commodity items.
Photos are indeed different from consumer commodities. However, if you are confident in your craftsmanship and write the warranty to exclude the customer's accidents, and limit the warranty to a replacement print and re-assembly of frame, then it should cost you very little to offer a warranty or extended warranty service and it would be mostly profit. Word of mouth is an important part of marketing and you need to stand behind what you provide. The notion that you did this using archival processes is apt to be considered b.s. by the consumer in the same way "green" or ecological or "organic" is generously interpreted by marketing-speak.