Magento – Add Product or Coupon to Cart via Link Not Working

This has worked for me on CE 1.7.0.2 (with sample data):

/checkout/cart/add/product/126?super_attribute[525]=100&super_attribute[272]=22

NOTE (this puzzles me a bit):
There is a difference between calling:

/checkout/cart/add/product/126?super_attribute[525]=100&super_attribute[272]=22

and

/checkout/cart/add/product/126?super_attribute[272]=22&super_attribute[525]=100

I mean the order of the super_attribute parameters is important. After calling the 2 URLs above I ended up with 2 cart lines of the same product with the same options. one looked like this:

Size Small Color Green

and the other was

Color Green Size Small

I guess if you add the products to cart via URL you should keep the order of the attributes as shown in the product view page for consistency.

I've had a similar issue; to address this, I created my own module and extended Mage_Checkout_CartController and overrode the addAction method. As you noticed the addAction method contains the following code:

public function addAction()
{
    if (!$this->_validateFormKey()) {
        $this->_goBack();
        return;
    }
    ...
 }

Either disable the validateFormKey completely in this method, or have your affiliates add a query parameter (I used affiliate=1 in my example below) to their URL that you can check against so you don't disable this feature entirely.

(roughly)

private function _validateAffiliate()
{
    $affiliate = $this->getRequest()->getParam('affiliate');
    return $affiliate == 1;
}

public function addAction()
{
    if (!$this->_validateFormKey() && !$this->_validateAffiliate()) {
        $this->_goBack();
        return;
    }
    ...
 }