If I buy a couch online, how would it feel if the retail website has a optional 15% tip for the delivery guy opted in by default in the checkout flow. And, provide an option to modify the 15% in case your couch was delivered late or damaged. Somehow, this workflow is widely accepted when its food delivery :)

No idea, why is it expected to tip delivery persons for some commodities while its not expected for some other :) Same thing, its expected to tip a driver for a shared lyft/uber but not expected to tip a bus driver. It's just funny at this point :)