There is not a Carrs Fuel station in Seward, but if you spend more than $50 on groceries at the Carr's grocery store here, you will still get your $0.10 off per gallon at any Carr's station. The western side of the Kenai has no Carrs fuel stations either. As far as I know, you'll only be able to get Carr's gas in Anchorage or Eagle River (the closest suburb north of Anchorage).
In Indian, Alaska (between Anchorage and Girdwood) is an Essential 1 station (on your left, heading south). You won't get a discount, but they do have pretty cheap gas considering. It is much, much cheaper than the Essential 1 in Seward.
FYI: Gas in Seward is $4.95 and diesel is $5.58 - at all 3 stations, Chevron, Shell & Essential 1.
On edit: I don't know if y'all are familiar with
http://alaskagasprices.com/. You can enter the location you want, or the station you want throughout the state and see the latest gas prices. Might be helpful.
Tammy