Maybe try your local community college? I didn't know what Straighterline was so I googled, and under the "people also ask" tab, I only see people asking about cheating on exams so that seems like a bad sign.
Then again, I doubt adcom will know much about the Straighterline program and might just pass it off as another college course.
If I was in your shoes and just need physics credit, I would just take it at my local CC or other accredited institution to be safe. I wouldn't want to jeopardize an application cycle over a physics class.