For PayPal payments, we offer both manual (one-time) PayPal payments and PayPal Subscriptions, which create an automatic recurring subscription.

 The option to pay by PayPal Subscription won't be visible on the invoice unless it's on or before the due date of the invoice,.  If you have passed the due date on the invoice, you can still submit a manual PayPal payment and create a new PayPal Subscription on the next invoice before the invoice due date, after it gets generated. 

If you don't see any of the PayPal payment options, simply choose "PayPal / PayPal Subscription" from the invoice payment method drop down menu on the top right corner of the invoice  to change your payment method.  The PayPal one-time payment button as well as the PayPal Subscription button, if applicable, will be shown.