3D FlipBook - Licence increase from 3 to 7 sites

Hello, I have a 3D FlipBook WordPress Plugin x3 3 sites license One-time and I am wondering if possible to increase to 7 sites and in the future 30 sites, just paying the difference. I paid 239$ for 3 sites, so now for 7 sites should be 110$ extra to reach the amount of 349$. Looking forward to hearing from you soon. Regards