An interruption during Bircat Ha-Mazon

Q: I work in a mini-market by myself. I am occasionally in the middle of the Bircat Ha-Mazon when a customer comes in. Is it permissible for me to put the items in a bag and take the money without speaking? When I signal to wait, some people get upset, throw down the items and leave.
A: If there is no choice, it is permissible to stop reciting the Bircat Ha-Mazon without speaking, take the money, print a receipt, give him the item and nod your head politely, and then continue reciting the Bircat Ha-Mazon. If you are after "Ve-Al Yechasrenu," it is permissible to speak since it is after the fourth blessing and what comes after are later additions.