Maby you haven't received anything from Subway because they fell off their chair when they were laughing so hard. Seriously it is amazing how people can't seem to figure out a simple coupon.
From your report it says if you buy a Sub and Drink you get another sub at equal cost for free. You bought a 6-Inch Meatball sub and a drink. The 6-Inch Sub was probably about $3.50(a 6-Inch is not just half the cost of a 12-Inch). The Drink was proably be around $1.50. When you add in tax being charged $5.50 for that would be correct. Had they charged you for the 2nd 6-Inch Sub your total would have been around $9.