Hi Dick. Yeah had a go a while back, and forgot about it. Then got an email saying I'd made a sale- about £1.80 or something. I ain't complaining! It's a low commission but it works in conjunction with Google - showing the shirts on a search.
I believe Merch by Amazon pays more, but takes more effort.
Loads of good tips on YouTube. A guy called Passive Owl has a bit on it. Google Trends is good for coming up with ideas, and seeing what's hot.
Hope that helps