My best selling gig is proofreading articles. Most of them are written by writers who do not have English as their first language.
I have found that Grammarly Premium does not find all of the errors and even corrects some errors incorrectly. If you are using Grammarly Premium and English is not your native tongue, you could be delivering content that is not written well by a native English speaker’s standards. Thus your score in “service as described.”
EDIT: I just looked at your gig. You promise 0% mistakes. I would be careful with that promise. A buyer could prove they found one error and then CS would let them cancel the order.
Also, in your article titled, Top 10 Rasberry Pi Projects, I found an error in grammar in the second sentence that I think Grammarly Premium would have caught.
Edit # 2: Yep, I just typed the sentence into Grammarly Premium and it found the same error. Oh wow, as I read on I find many more errors.