That assessment is an audit to make sure you are not compromising their users' data or doing nefarious things. It's conducted by an external reviewer who charges the fee.
It's now part of passing the test to be a Gmail / Gsuite extension. I doubt there are viable alternatives if you want to be integrated to the point of feeling native to the user.
Maybe a browser plugin?
Call it a cost of doing business, there are larger costs if you are successful anyhow, so it's relative.
