Often in shops/bakeries/etc within NRW, Germany, a common way of asking Is that everything? is to ask Das war's? as a question. (Das war's comes across as if it is pronounced as Das was, though this question suggests that das war's is what is in fact said.)
In my experience as a German learner, I often reply with Das ist alles. but I've never seen another German speaker answer in this way. I often observe that when no additional items are required, the question Das war's? is often typically answered with Das war's..
Am I correct in my observation? What is the standard answer when there are more items to be added to the purchase?