Since Printful is your printing partner of choice it may assist to contact them and ask why your items are not connecting and how to do so. I haven't used the site but this is likely a simple fix or missed clicked box on their site. And since they are collecting a fee for your printed work they should want to help.
Disappointed there weren't more responses to this question. The link provided above is no help. I am technically connected to Printful but my products are not showing up without manually adding them. This leaves me wondering will they be fulfilled when an order is placed. Printful was no help. They said contact Etsy! Ugh!
