Cookbook
Recipes for the things a real shop needs and Merx does not do for you.
The guide gets a shop running. These recipes go past that, to the features every shop ends up wanting: selling a product in several sizes, charging for shipping, giving a discount, saving a product for later.
None of them are Merx features with an option to switch on. They are patterns built from the parts Merx does give you — list items with a type, hooks that fire when the cart changes, and pricing rules. Read one that is close to what you need and adapt it.