If your Walmart carries the single Robitussin To-Go packs, you can score one for free plus get around $1 overage with this new $2/1 any Robitussin product coupon! You can usually find these in the checkout aisles or on endcaps and cost $1-$1.25.
Click here for coupon (zip 99218, under Health Care)
NOTE: This is an elusive product. Not many stores carry it, but it's nice to have on hand in case you come across one at a drug store!