How should we price our plans and be profitable?

Hi folks. We are about to launch our customer support SaaS product. We want to offer several plans, but we want them to have a flat price. Not based on usage or per seat. We think this will be one of the selling points, as pricing in similar apps is a bit confusing.

Our problem though is that the costs may be variable depending on the usage (emails) and storage (attachments) and traffic (resources to scale).

Do you know of any proven techniques to do some kind of analysis that will help us choose plan prices that can leave room for being profitable taking into account that some tenants may underuse and some overuse their accounts? Meaning that the balance in the end will leave room for profit?