Answer: Promo codes are case sensitive, so the promo code in the URL must match the code entered in the ticket type configuration. For this reason, we recommend avoiding mixed case for the promo code in order to minimize any issues around capitalization.
Are promo codes case sensitive? - Blackbaud Knowledgebase
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For the manual code you created, the "discount code isn't valid for the items in your cart" error should only appear if there were restrictions on the code that the cart contents don't match, such as minimum quantity of items or minimum purchase amount.
Discount Code is not being applied - Page 3 - Shopify Community
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A coupon code is always free – If you ever come across a website that asks for some cash to give you a 50 percent coupon code, get away from that. It's a scam. A legitimate site that cares about their customers and offers coupon codes will never try to sell them.
Should you trust a coupon code website? | AZ Big Media
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One of the biggest disadvantages of the Shopify platform is its inability to combine multiple discount codes. There is basically a hard limit of one discount code (manual or automatic). ... There is basically a hard limit of one discount code (either manual or automatic one) applied per order.
How to Combine Multiple Discount Coupon Codes in Shopify
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