Adding a PayPal button is very easy using a SubscriptionBoss shortcode; you simply place the following text within square brackets
subb-button subscription="mysubscription"
where “mysubscription” identifies the subscription
The default choice of button is selected in SubscriptionBoss Admin Settings however you can supply your own button by specifying the button parameter as follows:
subb-button subscription="mysubscription" button="http://images.mysite.com/my-paypal-button.com"