If you've selected a one-time purchase, you won't be "locked" into a monthly subscription. Rest assured that you don't need to commit to a monthly subscription unless you choose to. There are no recurring or hidden charges at all.
For customers who are interested in getting onto a subscription plan, so you don't have to keep reordering monthly, please select the subscription option on the product page. On the cart and checkout pages, you will see a description like "Deliver every month" for that product.