We've also created a license FAQ which answers common questions about FusionAuth usage. I think this is the relevant section:

I sell downloadable software that contains FusionAuth. What type of license do I need?

And the answer:

You will need a reseller license for FusionAuth in order to resell it to your customers. This license is usually charged per customer rather than per monthly active user. You should contact our sales team to discuss licensing options. They can be reached at sales@fusionauth.io.