Is it possible to buy just more signing licenses and/or does Personal Pro come with API?

It looks like the only option to have more than Personal Pro Box Sign: 15 docs for e-sign per month; Unlimited templates, is to go with Business Unlimited eSign signatures, 50K API calls per month. 

Is that right? 

We are a small business and don't need 3 licenses to sign leases. Also for $45 is a lot more than docusign at Standard $25 /month per user but I guess that limit is 100/user/year. so better if some months over 15 but worse per year