As a followup, it turns out that this wasn’t sufficient. I had to use the orange lion menu to set “Shields Down” for a total of three sites:
The site that was originally trying to sell me something
The third-party payment processor site (in this case, paddle.com was the site, and I was lucky to figure this out)
If PayPal, then paypal.com must also be set to “Shields Down”
Ideally, Brave would be able to apply “Shields Down” recursively, to all sites that are referenced from the main site that the user is viewing. Then, complicated multi-site things, such as this payment workflow, would work properly in Brave.