Thanks @slash1andy, highly informative! I appreciate your time, link, and info.
We’re in Canada, so that may account for some of this.
Just to be unambiguous: prior to Stripe activating the gated feature on our account, we saw “Received unknown parameter: level3” errors on most transactions. The plugin didn’t learn level3 wasn’t available and insisted on sending it every time, triggering an error. This was unrelated to the level3 totals mismatch.
I noticed the 400 errors from the API logs on the Stripe dashboard.