It is not sufficient to say you are unhappy with the final product. This is acceptable, but it contradicts their earlier promise. You have only three days to review your paper before you cannot get a refund. This is too short of a time, especially as they require written proof such as a scanned copy your instructor's feedback.
On writers' pages, there are hundreds of GradesFixer testimonials. Most of them are positive but only a few words in length. It is however very suspicious that TrustPilot and SiteJabber have not been included in the reviews. Although there are GradesFixer review reddit topics, these are created by the company and have no comments.