Create/Update a commerce order using parameters passed via the url
As far as I know, the only way to do this is with a custom module. It should implement hook_menu() to accept as many parameters as you need, and invoke a callback that uses the Drupal Commerce API like in this example.

[EDIT] There are three modules that may help you do this if you don't want to create a custom module yourself:


Rules link. It attaches a link to entity displays that triggers a rule. It passes the entity to the attached Rule
Rules link event. In some ways similar to Rules link, but from what I can tell, you can actually use the SKU (or other parameters) as an argument in the URL link, accessible to the Rule through a token [path:last] and so forth. Main difference between Rules Link and Rules Link Event here. 
Commerce cart link. Similar to Rules link event, but works with ctools to create a token for the link, which increases security.