Present other competitive offerings to leverage a better deal from PrimaryIO. Make sure to emphasize that other vendors have provided lower quotes or better offerings, which could motivate PrimaryIO to compete for your business.
Negotiate to have overage fees waived entirely, or at least minimized. By highlighting usage trends, you can argue that consistent usage reflects the need for better pricing strategies.
Request that language regarding auto-renewal be removed from the agreement. Emphasize that having to provide notice ahead of renewal puts you in a vulnerable position and that you want to maintain flexibility.
Propose your participation in case studies or serving as a reference as a condition for better terms. This could provide PrimaryIO additional marketing value if they offer competitive pricing.
If applicable, stress the increase in usage and justify the request for a lower rate per user. Emphasize that as your team scales, the pricing model should also become more competitive.