Vanilla Javascript Firing Twice
I don't think you can force javascript to only run once. In addition to big pipe, any lazy loader, ajax form update (consider entity reference fields), or other ajax functionality on the page can trigger javascript to run again.

This is the issue that jQuery.once() addresses, and the reason it is used. It adds a class on the element, then executes on elements that do not have the class and adds the class. I expect you could reproduce this with something like the following.

const usaButton = document.querySelector('[data-sales-switcher="83"]');
if (!usaButton.classList.contains('node-page-filters-sales-once') {
  usaButton.style.border = '6px solid orange';
  usaButton.classList.add('node-page-filters-sales-once');
}