How do I create a promo code or a discount for products in an online store?

To create a promo code:

• Go to Site Settings → Payment systems → Promo codes.
• Enter a discount code. It should be a combination of letters and numbers, up to 12 characters in total.
• Set the discount value: it can be a percentage or a fixed amount off (the currency of your promo code matches the currency you selected in your shop).
• Set the number of promo codes you'd like to offer (if the restriction is not necessary, leave the field empty, or set it to 0).
• Set an expiration date and click Add.
• Then go to the page with the cart, open the Content panel and add the Promo code field. It will appear in the order form.

Read more about creating promo codes: http://help.tilda.ws/online-store-payments/promo-codes