Try Teezily, I had a few things printed by them and the quality was quite good. They also have an app that directly integrates into Shopify, which you might also find useful.
I must confess that my foray into custom apparel didn't last too long, I realised that I was just chasing money and that I really didn't have any place anywhere near fashion! So I don't know how they might cope with volume, it was never a problem for me.
I must confess that my foray into custom apparel didn't last too long, I realised that I was just chasing money and that I really didn't have any place anywhere near fashion! So I don't know how they might cope with volume, it was never a problem for me.