I've seen this several times online. Especially amongst product creators who provides a product where the client most likely will have questions after they've placed their order and received the product. - They basically charge anything between $5 and $50 extra for this so-called prioritized support. It's a bit similar to the "instant delivery" in my opinion. You can pay $5 extra or whatever to get your order delivered quickly.
This is sort of the same thing, but it's with actual support. Basically, what these product creators offers are a faster response on any question you might have, if you pay the extra for "prioritized support". - However, you will never know if these people actually do offer prioritized support or not, as you won't be able to see their responses to other clients.
So, long story short. Do you think it's fair or okay to offer something like this?