A $0 item or a 100% off coupon will not work with Monto subscription plans and will not initiate a subscription for the customer. You will see a subscription error in your orders page in Monto.

This is because there are no credit card details that are captured if a customer uses a 100% discount coupon in a Webflow e-commerce store.

There needs to be a purchase at point of sale for at least $0.50 or else Webflow automatically hides the credit card portion of your checkout page.
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​If there are no credit card details stored... there is no way for us to create a subscription for them.

The way to get around this is an extended free trial on a subscription plan or simply refunding the invoice in your Stripe account for the subscription that you want to be free. (You can also set a very long free trial period on the individual Stripe subscription if you wish as well, after refunding the original payment.)