I am trying to create shopify app with rails. I want to add functionality in the cart page, to select different store location, to set custom shipping address. My questions are:
1. What is the best way to do so? script tags or writing to template file in theme?? Or are there other preferred way to do so?
2. How do I make my elements blend in with their theme?
It depends if your functionality needs an app. Will you be utilizing the Admin API? If so, this would be through an app. If not - you may just be able to make theme changes as authenticated would not be an issue.
In regards to blending with the theme, matching CSS and theme elements in the same way you would develop a theme.