Headaches with prepostseo plagiarism check

I was offered bulk article order from a client. He insisted the articles to feature no plagiarism. He checked the articles on prepostseo plagiarism tool and reported there was too much plagiarism. I used the tool to correct my articles. It shows plagiarism whatever I do as in corrections.
This one time, it shows 4% plagiarism in a 400 word article. When i delete the entire sentence that's mentioned and check it again, the result is 18% plagiarism. Smallseotools, show 0% plagiarism for the text.
There was this extreme case. The site actually indicated 0% plagiarism on an article. When i check it again after few minutes, the plagiarism is 18%.
What is a decent free plagiarism check me and client can agree on.