In addition to being able to add taxes or the return URL in the Paypal button edition. You can also specify these values globally

In your Craft CMS dashboard go to PayPal → Settings→ Taxes

To add taxes globally for when create a new PayPal button fill in the following fields:

• Discount Type: Rate(%) or amount.
• Tax (If Discount Type if Rate, Tax need to have a value between >0 and 100)

In your Craft CMS dashboard go to PayPal → Settings→ Defaults

Here you can set up global values for  Return URL, Cancel URL and Currency for when create a new PayPal button. Fill in the following fields:

• Global Return URL: Redirect buyers' to this URL after they complete their payments
• Global Cancel URL: Redirect buyers' to this URL if they cancel checkout before completing their payments.
• Global Currency: The currency of the payment