How to setup one time purchase option?
By default, when you create a subscription group, the products you add to it will be available for both one-time purchase and subscription. If you want to sell a product as a subscription only, you can enable the "Only sell this product as a subscription" setting on the product page in your Shopify admin. If you do not want to sell a the product as subscription only, ensure that setting is disabled.
For more information, please visit Subscription-Only Products.