Yes. When setting up the shopping cart price rule, you'll see a drop-down for "Coupon". Select No Coupon" from the drop-down. This will enable the discount t to apply automatically, without the use of a coupon code.
In the Actions section, toggle Apply to Shipping Amount to "Yes".
In the Apply the rule only to cart items matching the following conditions (leave blank for all items) area, you can set logic that dictates that this will not apply to your products. For example, you could dictate that the discount only apply if the Category is Not one of (your active categories), or if an attribute has a value which none of your products have, or even if the product price is not greater than $0. Depending on the makeup of your product catalog, you could choose how to set this to stop the rule from applying to products.
You can test this with a Percent Discount.
Best of luck!
