Just to give my 2 cents on this:
It's a viable way to make money. I sell t-shirts on Amazon, Etsy, Ebay and other sites and have recently been accepted for Merch for amazon on october 13th or so. I only started adding shirts on the 15th (2 per day max). Right off the bat, I started making a good amount of sales. I thought I was doing average numbers but if OP made 300$ over several months, I guess things are better than I thought.
If it helps at all, here are some numbers for october (16th to 31st because I just started selling).
Also, thanks for the tip about having to fill 80% of the possible listing capabilities to upgrade to the next tier. I had no idea. I just started uploading some more to try and fill up my slots. Been at 25 since the first week. I had tier'd up on the first week and have been waiting for the next ones forever.