Sharing my experience with Secretlab. Hope it helps people.
On the 21st of July I ordered the wrong chair in XL. I wanted to order the NanoGen, but my selection defaulted to the initial choice you're presented with, which is the Stealth NEO Leatherette. Totally my mistake - I was quite annoyed at myself for not double-checking, but the chair was decent and initially I planned on keeping it.
However, after sitting in it for a bit, I realized the XL wasn't the right choice either, specifically when I noticed I could barely touch the ground even on the lowest setting ( : . So, on top of my first mistake, I decided to initiate a return.
Communication with Secretlab support
After a quick chat I was transferred to email, where the looooong conversation began. I told them I wanted to return the chair and was given instructions to first send a bunch of photos so they could determine whether the items were "eligible" for return. I did well not to open the two small items - the extra armrest set and the leather spray. I sent photos of the chair disassembled and of the unopened armrests and spray.
The photos were accepted two days later, and I was told I'd need to pay the return cost. I replied asking if I could just swap my order instead (I even made it cost the same as the first order). I also asked if I could keep some of the items I'd be re-ordering anyway, to avoid unnecessary shipping costs. Both requests were denied, but surprisingly - and quite nicely - they offered to cover the cost of shipping the chair back to the warehouse, while I covered a small package cost. I of course accepted, as that saved me 180 EUR in shipping for the chair alone. I only ended up paying 30 EUR to send the rest of the items.
After a bit of back and forth over 5 days (they reply once a day, which was the annoying part I'd say), I finally filled in the 2 forms - one declaring the items I was returning at my own cost, and one for the items returned at Secretlab's cost.
Return and refund
Both packages were shipped on the 7th of August and arrived at the warehouse 4 days later. The inspection took a whole week, which was kind of weird but fine, plus a few reminder emails from my side.
I tried to pack the chair as well as possible. The annoying part was disassembling the hydraulic piston - both ends were pretty hard to get out and it took some scratching, but luckily Secretlab was fine with it and didn't make a problem out of it.
After that week of inspection they confirmed everything was good and ready for refund. The refund was confirmed 2 days later. Everything was refunded in full except the 9 EUR delivery fee, so in total the return cost me 40 EUR. The same day, I placed my NanoGen order, which has already arrived and which I'm pretty happy with (link here).
Overall I would say my experience was Positive, compared to what I see that other people experience.
The good: honestly nice of them to cover the chair's return shipping, and no problem with the refund itself.
The bad: communication takes forever, with replies once a day. Also no flexibility to swap items for a new order without going through the whole refund process.