What you are allowed to do
Put it on a commercial site, a client site, behind a login, on as many pages as you like. No licence to read, no account to make, no key to rotate.
The only ask is that the "Powered by" line stays.
Not an iframe
The calculator becomes real elements on your page, inside a shadow root. It sizes itself to your column, so there's no fixed height to guess at and nothing scrolling inside a box.
The shadow root cuts both ways. Your CSS can't reach in and break it, and its CSS can't leak out and touch your page.
Where a script tag is stripped, Squarespace and Wix code blocks among them, the builder still offers an iframe. For Reddit, GitHub and forums it gives you a linked image instead.
It will not break on you
The frame points at a URL, not a snapshot, so corrections and new features reach every placement without you touching the code again. That's also the risk of an embed. If this site goes down, the box goes blank. It has been up since the move to Cloudflare and the calculators are static files, which is about as few moving parts as an embed can have.