I've never actually heard anyone say 'Please come again'. The only time I've heard it is Apu from The Simpsons, and he's in the US. I have heard the occasional 'have a nice day', but normally if the cashier says anything, it's just 'thank you' or 'thanks', and a 'see you later' or something similar.
In some shops, they will have a sign over the exit which reads 'Thank you for shopping at X. We hope to see you again.' or something similar.