What's the easiest way to pre-fill a shopping cart in Magento?
There is the CartController on the Checkout route, which has an "add" method, allowing you to do stuff like:
http://<shopurl>/checkout/cart/add?product=1
But what to do when you need more items in the cart? There is an "addGroup" method, but that only takes into account previous order lines.
When digging into the Checkout/CartController/addAction, there appears to be an argument "related_product". It enables you to do this:
http://<shopurl>/checkout/cart/add?product=1&related_product=2,4
Downsides are you only get a message about the main product added, and for the related products you cannot specify quantities. Upside is, you can populate a cart like this with several items at once without even touching Magento code.
If you want to add more items of the same, just repeat the id in the array
