Fair offer for a photo to be used on a T-shirt?

I recently saw a picture on imgur that I think would be great for a shirt for my fledgling print on demand business. I want to ask the person who took the photo for permission and compensate them but I am 99% sure they are just an amateur photographer who took a picture on their phone so I'm not sure what to offer them to use the image. (as I am essentially an amateur T-shirt creator myself)I didn't know if anyone here would have an idea of what would be fair. A % of sales? A flat fee? Either $x up front or $1 out of every sale type of setup?I asked this in the print on demand subreddit but think this is the better place to ask to get an idea of what photographers think is fair. via /r/photography https://ift.tt/2MS4qGF