Let’s be real here…
It is essential to understand that as a user, the responsibility of being aware of a SaaS’s billing terms and conditions prior to making any purchase rests solely with you. It is not reasonable to overlook these terms and then attribute the resulting consequence to the SaaS.
The “terms and conditions” and “FAQs” are provided on the same page where is the “Subscribe” button. Taking a few moments to read these could have prevented the misconception you’re currently facing.
While we can acknowledge that user alerts for rushed purchases could be a beneficial option, this does not absolve the user of their responsibility to understand what they are purchasing.
Regarding your point on upgrades, each plan indeed offers additional services or features. This could mean a new feature, even a new SaaS in one feature alone, or a new quota limit.
This is the essence of an “Upgrade”. The stacking policy simply means different plans add on to each other with the added services, features and quota.