A few days ago I launched a free app for Shopify. The app automates the process of sending customers tax receipts for their donations to your non-profit Shopify store. Its a pretty cool little app - I think it is a really good example of how to build a simple piece of automation using webhooks. I fully open sourced the code here: 

github.com/kevinhughes27/shopify_tax_receipts

so have a look!