Ask HN: Alternative to OAuth for Gmail Automation?

I have created a product in the email automation space and Google's OAuth process finally got back me and told me I need a $15,000 to $75,000 assessment because we make requests to an external service (our own service).

Obviously as a side project/business a large fee isn't going to work considering the product makes zero money at the moment. Are there alternatives? Would it makes sense to stick with IMAP all the way?