I don't have any experience with coupons in the shop myself, but there are instructions here.

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How to activate Discounts and Coupons in the e-commerce shopping cart

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• Set up coupon codes and discounts: now it's time to put your plan into practice and setup discounts and coupon codes on your WebSite X5 store. You can set discounts/coupon codes, fixed or in percentage, on:
  • single products
  • the order total amount
  • shipping costs too

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>> https://guide.websitex5.com/en/support/solutions/articles/44001796967

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If you have any further questions, Incomedia employees (online during office hours in Italy) or other shop operators can help.