The documentation for shopify is horrendous, there are filters and various other undocumented features that I found in the Liquid template documentation.
Hard to find out how to customise certain areas of the shopify front end. Such as the customer login areas.
Check out Process
This is the worst part, you cannot specify your own HTML for ANY of the checkout pages. You have to rely on CSS to try and style these pages. What makes it even more of a headache is Shopify only gives you terrible markup, nasty use of tables, ID's and classes through out.