How do you say it in English? Thanks
What are the words used on a sign that says that for more extra little money, you can get one more item after you buy something at stores? What is the idiomatic way to say it?
special offer to additional purchase? Thanks

Question

How do you say it in English? Thanks
What are the words used on a sign that says that for more extra little money, you can get one more item after you buy something at stores? What is the idiomatic way to say it?
special offer to additional purchase?  Thanks

Answer

Typically a sign would just say "special offer" in large text followed by an explanation of the offer in smaller text.  It would not be uncommon to see an advertisement for the main product with a section saying "bonus" or "bonus offer" following the regular advertisement.