Depending upon the plan you have chosen, you can use this product on multiple websites If you have purchased the PRO plan then there is no limit on number of websites on this lifetime deal.
All, you need to do is create separate Campaigns for each of your website and then under each Campaigns you can create unique Notifications for only those websites.
PRO Plan: Use the product on unlimited websites.
PLUS Plan: Use the product on up to 50 websites.
Free plan: Use the product on up to 5 websites.