Backstory: I recently started an e-commerce subscription company and we were sending out hand written welcome cards. We noticed the cohort that received the cards has far less churn than those that didn't. The cards were very time consuming so we stopped sending them. I tried a quick google search to find a service that provides something like this but I didn't see anything. 

The Idea: Website where you upload a csv and print personalized welcome cards for customers.

Deep Dive: You would build a website where users could upload a csv of their customers and print out a customized thank you/welcome notecard. You could offer different templates and styles. Additionally you could support merge fields, allowing users to upload more than just names and emails, they could create a truly custom notecard message.

Validation: Very similar to wedding place cards, you could find a niche in the e-commerce space.

Monetization: You would charge per notecard generated. This would scale as your customers grow.

Steps to Launch: I would keep it simple and have one template and only allow the name field to be configurable. Ideally you would work with an existing e-commerce company or customize this for your own.