The One time purchase option is by default turned on- for the Ongoing subscription widget.
If you are only seeing a subscription option on a product page and you'd like for the one time purchase option to be displayed, you'll need to control this on the Shopify Product page.
On the shopify product page, if you would like the one time purchase to be on, it will look like the below photo.
From Shopify, click on Products
Navigate to the Product you want to change.
Navigate to the section Purchase Options
Uncheck the option "Only show this product with these purchase options"
Click Save