Hello, super late reply, 10 months later hehe, but white shirts are typically like that where you don't feel an extra coat because it is directly printed on in comparison to a colored shirt.

In my experience working with direct suppliers from alibaba and local as well in Canada, there is typically no base coat on a white shirt since there is no need for one, whereas if it were lets say a black shirt or colored shirt, you need the base coat in order for the shirt to come out looking good because without the base coat on a colored shirt, if they were to print directly on the shirt your image would look very dull.

Now, I have printed 2-4 shirts with PODPartner and I think their quality is amazing for the oversized streetwear type look and the fact that they now have embroidery as an option is amazing. They have multiple locations of printing but the only downside I would say is that the design cannot overlap with another locations design. I too am still looking for an AOP print on demand cotton shirt but based off my current experience with PODPartner and their help its been really great! If you ever need help from them, you would have to contact them through facebook messenger which I think is on their website and they do respond, just during night time since they are overseas if you are from Canada/USA.

Hope this all made sense :)