1. Create the Website Payment Standard Button at Paypal

2. Copy the HTML code of the button

3. Login to Joomla

4. Go to User Manager and edit your account

5. Select No WYSIWYG as the User Editor in the Parameters tab

 

6. Edit the page and paste the button HTML code

7. Verify it

 
 
 
Done =)

Reference: How to add PayPal buy now buttons?