Can my customers use Cash On Delivery to pay for their subscriptions?

You can use Cash On Delivery (COD) as a payment gateway if you are using Recurring invoice subscription rules. Recurring invoice subscription rules allow you to use any payment gateway in your shop.

On the other hand, you unfortunately can't use Cash On Delivery for auto-charging subscription rules. This is due to the fact that in order to use Auto-charging subscription rules, you need to use either Shopify Payments, Bankful, ShopPay, Apple Pay, Google Pay, Authorize.net, Stripe or at least PayPal Express as a payment gateway. You can see this requirement on Shopify's official documentation here: https://help.shopify.com/en/manual/products/purchase-options/subscriptions/setup#eligibility-requirements