Hi,
Does anyone here offer a proof-reading service that just checks grammar?
It sounds like you are looking for some kind of commercial service where you pay a fee.
Here are a few comments on how the Forum operates, from my point of view.
The Forum is a teaching-oriented environment, where the focus of correcting is to help people to improve their English. All the helpers are volunteers. I prefer to correct short passages, although sometimes I have time to correct long ones. I am very reluctant to correct long passages repeatedly for the same person. If a sentence is awkward and unnatural, but grammatically correct, I always point that out and help the writer to correct it.
Best wishes, Clive