I’m going to offer a slightly different take on this - and regular forum contributors will no doubt think I’ve gone crazy, as I’m quite hardline usually.
As a fellow proofreader I would suggest taking the pragmatic approach on this occasion and simply completing the order. Why? Let’s be honest, the time difference between proofing 500 words and 850 words is maybe 10 minutes tops - maybe only five minutes.
While I believe it is incredibly important to stick to a pricing structure, sometimes it is also best just to accept that some buyers are stupid or abusive, and it’s therefore in your best interests to get the job done and move on as quickly as possible. Even block them afterwards if you choose.
For the sake of an extra five or 10 minutes of your time and a polite message with the delivery saying “the gig covers 500 words but I’ve done the decent thing and proofread your full 850 word article” - you can save yourself so much time, hassle and potential lost income.
Again, why? Well, if you try and get the buyer to cough up the extra money, it’s still going to take you those five or 10 minutes to compose and send a message to your buyer. If you or they then choose to cancel the order, this is going to impact not only your completion rate but also (according to Fiverr forum folklore) where your gig appears in search results - thereby leading to fewer new orders for some time to come.
Even as a hardline forum contributor, I believe the pragmatic approach is sometimes for the best. Just complete the order and move on. We’re talking five or 10 minutes of your time - not five or 10 hours. Yes, I know there is a principle at play - but on this occasion put pride behind you, complete the order and move on.