My '99 discovery 2 w/ 115k miles gets about 9-10 mpg in town and about 15 mpg highway. The town mileage is poor, should be 14-17 from what I understand.
EZ LUBE has a $66 fuel saver package special. Not sure what they do exactly but it says theres a fuel system cleaning service included. Is this the same as an engine flush?
Should I pay more for a complete flush or decarbonization? What should that cost?

 

Save your money.
First of all, your in town mileage expectations are way high.
You are only of by about 2 MPG on your current numbers. 12 to 14 are good numbers in town on the road 15 is average and some will get as high as 18 to 19 on long trips.
Yes decarboning will help you, but before you jump into some cleaning process, when and what was done on your last tune up?
Have you ever changed your 02 sensors, when were your wires last replace and with what brand?