I have hired a number of technical professionals in the past.
The bad news is that there isn’t a “magic bullet” cover letter out there. The good news is that you *can* write it yourself. Each cover letter is going to be different, since each job and job applicant are different.
Where the purpose of the resume is to get the hiring manager to talk to you, the purpose of the cover letter is to get the screener to look at your resume.
If you are applying for a particular job in response to an ad, use the cover letter to specifically match your skills and experience to the needs expressed in the ad. Use numbers to show how much money or time you saved or earned your organization by your efforts.
If you are applying to an organization to tell them you want to work for them and there is no specific job opening you are targeting, use the same technique. You just won’t be able to match your skills and experience directly to job needs. However, if you do your homework (research the company, use your network to find out where they need help, etc.) before writing the letter and sending the resume, you should be able to find something specific you can talk about.
Above all, don’t spend $230 to have someone do something you can do better yourself. It’s not worth it.