Not being able to say a word when you have to, but being able so say it when you don't have to

Not being able to say a word when you have to, but being able so say it when you don't have to Hello. As I have mentioned earlier, I work as a cashier. When I have to say "would you like the receipt" and "have a nice day" I block completely. Even though I say the words out loud atleast a 100 times before going to work, I still block when speaking to a customer. And here's the weird thing. When the customer leaves, I can say the words fluently again, but when the next customer comes, I block once again. I'm trying as hard as I can to pronounce the words, but my brain just won't let me. I was just wondering if anyone has a theory on why this happens and maybe some tips