I've always found AirBnB service charge to be a bit inconsistent. If you pick any listing, then pick a range of date, say 7 days, then 8 days, then 9 days etc. the only constant is that the service charge goes up, but how much it varies is highly inconsistent. For instance, you may find that for one 7 days it's £25, 8 days it's $28 (+£3), 9 days it's $30 (+£2), 10 days it's $35 (+£5), 11 days it's £36 (+1) etc., even though the room charge is exactly the same (so it doesn't look like it is a percentage of the room charge - it really looks very arbitrary).
I find this odd, but it is what it is, what really matter is the total and I can choose to go ahead and book or not based on that.
Now when you want to make a change to the booking, it is very much the same thing. You pick a new date, and it shows the new price. It even thoughtfully break it down again (room charge, service fees, and cleaning).
And this is where I encountered something very odd.
I made a booking that came to £187 initially. It's a very cheap stay, reflected by the fact that it's functional but exceptionally old. When I decided to change the date, the confirmation screen showed a subtotal of £177 (room and cleaning) and a service fee of £21, for a total of £198. I was a bit surprised that the service fee didn't change, and tested to find out that if I was to extend my stay by two days, the service fee would be £24, so it's didn't look like a bug. So I went ahead submitting the request for a change, which was accepted.
However, when I logged back to see my booking, I note that the total had come to £205, and in the breakdown, they charged me £28 in service fees. Now it's £7, not the end of the world when travelling the increase of £11 -> £18 is quite a bit as a percentage, and for £18, I would have gone with an "alternative" kind of accommodation which would've counted as travel experience. But it's really incredibly dodgy. Whenever I book something with a company I haven't before, and haven't heard about much, I tend to take screenshots of the confirmation screen to avoid any surprises, but here, they really got me by surprise. I sent them a support ticket, but the problem is that it is pretty hard to prove it, even though if I was to try to change another day, the screen will show that the service charge is £24. In my experience, service charge only goes up the longer you stay, something is amiss.
A few bucks might only leave a bad taste, but the fact that it is possible for them to charge a different value to what I agreed for is quite scary. I sent a message to their support (pretty well hidden on their website) but expect little out of it. From now on, I will screenshot every step of my booking / rebooking with them.