FWIW I've previously bought a few bikes from
Wiggle, two of which developed paint defects after between 12 and 18 months, (both were from the same manufacturer but were different bikes).
Wiggle sent me a bike box on each occasion, and once I'd notified them the bike was packed up they arranged to collect it.
Each bike was stripped down by Wiggle, the frameset was sent back to the manufacturer, and after inspection a new frameset was duly dispatched back to them. Wiggle then re-built the bikes each time and sent them back to me.
Everything was covered and despite the small hassle of packing the bike I was not put out in anyway what so ever.
I'd guess some other online retailers match this service, some may better it in somehow, and others may fall short of it, but you will need to return it to them one way or another.