Regarding PODs...

I have a few of question regarding running a Print On Demand business...

1. Can you upload the same designs on different retailers? Like for instance TeeSpring and Amazon?

And if you can only upload a design on 1 retailer...

2. What is the best retailer to work with in terms of flexibility, traffic (perhaps nobody will beat Amazon here), regularity of payments, etc.

Everytime I see a "POD" thread I think Proof of Delivery.

I need to flush out my DNS from the shipping days.

From what I believe this are all platforms that have their own marketplaces and lack integrations to e-commerce platforms like shopify and woocommerce.

I'd recommend posting on all of them, spread a wide net. This can work for organic traffic (if that'll ever happen). However, I'd focus on driving traffic to only one platform, preferably one with fast shipping and a higher profit margin.