There are a dozen apps that let you create a POD business. Depending on your location, you can search for the app on the app store. But here's how I'd search it if I was looking to start a print-on-demand business in the US: https://apps.shopify.com/...

It's really easy to do this with Klaviyo, assuming you're already using that as your email ESP.  Here's an help article for the same: https://help.klaviyo.com/hc/en-us/articles/360028872611-How-to-Create-a-Post-Purchase-Flow

Hey there, it's really easy to create bundles with Box Builder, one of the simplest apps to create bundles with hundreds of customizations. Give it a try and I'm here if you need help.