Best POD sites for 2018

Since 2015, Print-on-Demand ( POD ) services was the new trending for artists, individuals. Revenues from these POD platforms brought artists a lot of money.
How does a POD work?
a creative artwork
As an artist ( painter, graphic designer ) make a completed design, exported a PNG file, then he upload to POD sites with his registered account. He submitted the design for multiple products ( T-Shirt, Mugs, Phone Cases ...etc ) with a fixed price ( higher than base price for his higher revenue ). His account linked to his bank account for further payment. A customer found his design on a T-Shirt, she purchased it on POD site, paid for POD, POD shipped product to customer, and the bill printed. POD will share the revenue with that artist.
Which POD site still work well in 2018?
As far as I know, here is my recommended list:
Merch by Amazon (MBA) ( merch.amazon.com )
Redbubble ( redbubble.com )
TeePublic ( teepublic.com )
Zazzle ( zazzle.com )
Teezily ( teezily.com )
Teespring ( teespring.com )
When do I start this business?
Firstly, you can start this business at any time. But for MBA platform, you'd better apply right now, because the approval process will last 6-12 months. Other POD sites will approve your application automatically.
How much money will I get from this opportunity?
According to my story, I'm not a creative artist. I'm just a technical engineer, a amateur photographer. I started this since October 2015. I've just got 100-200$/month, sometimes only 40-50$/month. It depends on how good your designs, your artwork are.