I get stressed just THINKING about this, because I had a job that required counting everything up at the end of the day, not just money but other receipts and transactions, and if it didn't balance, I had to work out where the problem was. My job was behind a wicket within another business; the wicket closed at 5 but the rest of the business was still open. Customers could see the top of my head just over the huge CLOSED sign. And it was CUSTOMERS that continually interrupted, peering their heads over the top of the CLOSED sign.
"Excuse me, are you working?" (No, I'm here counting money for fun).
"Excuse me, I know you have your closed sign up, but could I still get...." (Well but of course, don't let a closed sign stop you, I'm not sure why I actually have it up to begin with).
"Oh! Closed at 5! But I just need....." (one sixty, one seventy, one eighty......argggghhhhh!!!)
I tried sitting on the floor with the money and inventory so that the customers couldn't see the top of my head. That didn't work. They'd peer over the top and see me crouching and clearly hiding, and say, "Oh, good, you're still here!! Are you open?"
When it's customers, you kind of do still have to be nice to them even when you want to throttle them. Well you have to be nice to employees too, but it's a little easier: gather them round, send out a memo, whatever, and say, "Please note, after 5 pm I am counting money, and it's not a good time to ask me questions. I am open to requests and questions from 9-5, but from 5-5:30 my focus needs to be on counting out the money and each interruption means I have to start over. Thank you for your consideration in not approaching me for unrelated matters in that half hour time period."