Experiences with print on demand products?

I have built a product using a print-on-demand API (lulu.com in my case), specifically around printing songbooks and utilizes their book binding service. I was curious if others have any insights or experiences they want to share. Preferably, things beyond adding your image to a T-shirt or coffee cup.

Compared to the SaaS apps I usually work on, there are some definite challenges:

• really slow feedback loop: hard to delight people over a few weeks vs. a few minutes
• shipping physical product: more up-front QA / testing fear
• lots of nuanced sizing/formatted challenges (unlike web, you can't just wrap text and scroll)
• legal/financial gray areas (am I selling a product? a service?, licensing?) -- I feel like much of this is very spinnable or open to interpretation
• security: not giving away print assets you generate
• pricing: how much markup is reasonable, maybe related to how much value you are adding

Some things that have helped me so far:

• generating PDFs as HTML (puppeteer) wherever possible, run on lambda for cheap
• a ton of extra dev/admin-only tooling: generate/view different layers of assets (cover HTML, cover PDF, internal PDF, preview)
• generating preview PDFs that you can view with a flipbook so users can get an idea of what it would look like -- very useful for debugging as well. there's an example at the bottom of the how it works page
• lulu + stripe sandboxes, thank god

I'm actually going to double down into that preview tool so I can show how easy it is to get peoples content into a flipbook so they can get a feel for what is possible (sales outreach).

Having said that, I built this particular product for myself first and have no regrets there. Constraints are fun, right?