IF A < If the first condition is true then...
blah1 < Do stuff
ELSE < If the first wasn't true, do this...
IF B < Check the whole thing again
blah2 < Do this if B was true
END IF < Finish the stuff that happened if B was true
blah3 < Do more stuff if condition A was false, ignore the check you just did with B
END IF < Finish
If A is true run blah1, ignore B
If A is false and B is false run blah3
If A is false and B is true run blah2 and blah3
IF A < If the first condition is true then...
blah1 < Do stuff
ELSE IF B < If A was false, check B
blah2 < Do this if B was true
END IF < If neither were true then finish
blah3 < Do this after all the checks
If A is true run blah1 and blah3, ignore B
If A is false and B is false run blah3
If A is false and B is true run blah2 and blah3
So in short, the difference is that in the first snippet blah3 (in this case the calculation and output) ONLY gets executed if A is false, in snippet 2 it gets executed whatever happens.
The calculation and output would only happen in the second snippet because in the first it gets ignored if you chose "Economy".