Absolutely -- it's a bit nerdy, but you can do it! Here are some examples to help you understand how it works:
In this example, an (active) Coupon Code will be automatically applied to the Shopping Cart:
https://www.hiramcollegebookstore.com/cart?couponcode=DEC2016?destination=cart
- The last bit (cart) is the "destination". It tells the Website what page to load after the coupon is applied.
Also, if you wanted to automatically add a Product to the cart using a link:
https://www.hiramcollegebookstore.com/cart/add/p22760_q1?couponcode=DEC2016?destination=
- The "22760" bit: is the item's node. (You can see this when you hover the cursor over the item's "Edit" tab, and look in the lower-left corner of your browser: https://www.hiramcollegebookstore.com/node/22760/edit
- The "q1": is the quantity to add to cart.