"xxxx it is then!", what does it really mean?

It is a summation that confirms an agreed-upon time, amount, or other article of negotiation.

In the following it is pizza:

A: What should we get for dinner?
B: How about pizza?
A: Pizza it is, then!

(A is probably going to pick up the phone and place an order at this point.)

We use 'it is then' in context with our totally corroboration. for instance: A:Where will we go, India or pakistan? B:I think we should go India. A:Okay, India it is, then.

here B decides to go India and A confirms it.By saying it is then,A also directs that we definitly go where you said.now B has no right to change the place.