クーポンって英語でなんて言うの？

Most commonly, in British English, we would say voucher. "I've got some vouchers for that restaurant" "I have drink vouchers we can use".
Coupon is more American English. You would hear it in the UK but not very often. Maybe at some airports where they give out food coupons for being delayed, but still you would most probably hear 'voucher'.

The general name for a small piece of paper or card that details a discount available for the holder, is 'Discount voucher.' A discount voucher usually will detail in percentage terms how much of a discount is available.For example, '20% off.' Such a voucher is different from a normal voucher which may allow you to have an item in exchange for the voucher ie without paying any money. It is popular in the UK to buy 'gift vouchers' for people for birthdays or special occasions.

A "voucher" or "Coupon" is something (normally a small piece of paper) that allows the owner to get money off of a product or entitles them to something like free delivery on an item.

The word voucher is used in British English. In the U.S., however, we say coupon.
In restaurants, grocery stores, and clothing stores we say "I have a coupon for this item."

Voucher and coupon mean the same thing. In some English speaking countries you would be more likely to hear the word voucher and in other English speaking countries you would be ore likely to hear the word coupon. In Australia most people say voucher.

"coupon" & "voucher" have the same meaning: a paper or card that gives you a certain discount on products.

"a statement of due interest"
"a voucher"
"a gift voucher"
"A statement of due interest" is a cut from the bearer bond when payable or presented for payment. meaning the interest rate of a coupon. A voucher entitles the bearer or person a discount for a particular product. A voucher can be exchanged for goods or services typically one given as a gift or offered as part of a promotional offer.