If your lens or camera still under Canon 1yr warranty, then send both body and lens to Canon. Canon will do it for FREE. Canon will ask for the original sale receipt from authorized dealer - no exception. Without receipt, they will charge about $200 US for this service.
They calibrated my 5D II + 24-105 for FREE, since my 5D II was under warranty at that time.
Would this work on refurbs purchased through Canon stores ... I did buy a 24-105L roughly one month back but, I find the focus to be bit off (on 5D3). Images come out pretty soft even under ideal lighting conditions (tried tripod/live view etc.). Tried focal that reported a +3 on Wide end but still not much improvement.
Typically canon offers 90 day warranty on refurbs. I did register for CPS so how do I initiate this request to Canon .. Sorry for the n00b questions.
I'm not 100% sure on refurbs item. My guess is you should be fine since your lens is till under their warranty.
Just tell them what you just told me - "you bought this lens through their site and it seems to be soft on your 5D III"
In my case, I brought my 5D II + 24-105 down to service center in Irvine, CA. I told them the lens is soft and asked if they can help. They did calibrated the lens to match with my 5D II.
The lens did get little better after calibration, but no where near my current 5D III + 24-70 f2.8 II or 70-200 f2.8 IS II.
My 2cents: the 24-105L is not a sharp lens. I tried 2 copies, no luck.