Of course you need to say please and Thankyou.
Why do we need to use those specific words? As yet nobody has been able to explain why the words please and thank you have to be used to make something polite, other than "manners" or "common courtesy", neither of which explain why those specific words must be used.
Why is "Is that everything for you today? Yes?There we go then, I've popped everything in the bag for you and altogether your total is £14.89. Are you paying cash or card today, sir?", when said with eye contact, a smile and pleasant tone impolite? Because there is no please or thank you explicitly stated so by some people's standards this means the cashier has been impolite ...
Why is "£14.89 please" said in a monotone voice and no other conversation during the transaction polite? Because they've used the word please so it must be polite ...
The words please and thank you add extra weight but they are not the be all and end all of politeness, especially in retail where there are many different ways to tell a customer their total and ask them how they intend to pay.
If I state a total ie "Is that everything for you today? That is £32.75 altogether then" then I don't need the word please as I'm not asking for anything, I'm merely stating a fact for the customer's information.
If I ask a customer for an amount then it is polite to add please but it is not mandatory to do so. "Is that everything today? That will be £32.75 then, please"
Both ways are polite and courteous, but only one explicitly uses the word please. When you work in retail and have this conversation 200 times a day or more (I work on the cigarette kiosk where, at busy times, I can be completing 2 transactions a minute as people literally walk up, ask for their cigarettes, I collect them, scan them on the till, tell the customer their total and they tap their card and leave) then you will not use the same wording in every transaction and some will naturally lend themselves to including the word please, some will not.