Printful is an amazing print-on-demand service provider specializing in apparel.
I don’t know where to start with printful, they’re amazing. Their prices are decent, they don’t use their name anywhere in the package they send, they ship in a timely as hell manner, and they’ll even throw your business card into the packages they send off.
I most commonly use printful with wordpress & shopify but it plugs into all sorts of sales channels, even facebook, ebay & amazon. When I get an order on my website, my website tells printful “hey, this dude bought something, please send it his way”. When they get that message they print the design I uploaded, send to my customer and send me a tiny bill. I pay the the bill they send me and keep the profits leftover.