F13 Works has a Shopify app you might be interested in.
Air Waves OnDemand Offers on-demand custom-printed apparel in a wonderful variety of styles and prices. Shipping from Ohio they offer the quickest turnaround and highest quality printing available. They are a big improvement over Printful for those of you with experience
All apps are free to download and include automated order processing, billing and fulfillment. You only need to place the product on your site with a mark-up and market them to your customer base.