What would B say in response to A in this case?
A: I've just bought two cabbages and five tomatoes.
B:
1 So I won't have to buy any vegetables this evening, will I?
2 So won't I have to buy any vegetables this evening?

Both are correct. In #1 the speaker asks for confirmation. 
 #2 is a just shorter version, in my opinion.

B could also respond the same way, with a question: "So I won't have to buy any vegetables this evening."  Just to restate the fact as they understand it.