Just wanted to add my experiences here for anyone who may find this thread in the future.
Zeko is my go-to for Chanel authentication. She is so lovely to work with for starters but what I appreciate most is how meticulous and thorough she is. She won't confirm authenticity unless she is 100% sure and she will continue asking for more photos until she has exactly what she needs. She also sends examples so that you know exactly which angle/lighting/etc. She's very easy to reach over email or WhatsApp, and I find it really convenient that I can add everything to a Google Drive folder and share the link. I've used her to authenticate 4 pieces so far and she always responds within 24 hours, even on the weekends. I feel her pricing is very fair because you also get a certificate. Also when I was inquiring about bag spa, leather surgeons told me that they consider Zeko to be quite reliable.
I've also used Authenticate4U and Lollipuff to get extra authentications on some of my items. I appreciate the quick turnaround with Lollipuff (I recieved an answer within a couple hours both times) however this also makes me think they aren't as thorough as Zeko. Authenticate4U never responded to my email, however they responded very quickly to my Facebook message, so I'd recommend contacting them that way. I got an answer within 48 hours (which I think was partially due to the timezone difference as I'm in Vancouver, Canada). I have been happy with all three services but I would trust Zeko's determination over anyone else's.
If Zeko were to say something is a fake, for me that's enough: it's fake.
If another service said fake, while Zeko said authentic, I'm not sure what I'd do as a Canadian, but if I were in the US, I'd send it to leather surgeons for spa service as they do free authentication since they only work on real bags. For me, Zeko + leather surgeons would erase any doubt in my mind.
Hopefully this helps!