next 15 upgrade turned every fetch uncached and my pricing api bill went 16x Caching

The default flipped. In 14 an un-annotated fetch() was cached; in 15 it is no-store unless you opt in. Same for GET route handlers, which used to be static and now are not.

Two ways to opt back in, and they compose:

export const revalidate = 60          // segment level
fetch(url, { next: { revalidate: 60 } })  // per call

And yes, the build output would have told you. The legend at the end of next build marks each route static or dynamic. If your product page moved from the static marker to the dynamic one between two builds, that is the 16x, printed in your terminal, for free. Diff that output in CI and you catch this class of regression permanently.

For anything that is not literally a fetch - a database query, an SDK call, an expensive computation - the fetch-level options do nothing for you. Wrap it:

const getPricing = unstable_cache(
  async (slug) => db.pricing.find(slug),
  ['pricing'],
  { revalidate: 60, tags: ['pricing'] }
)

The tag is the part people skip and then regret, because revalidateTag('pricing') from your webhook is much nicer than waiting out a timer.

Separate from fixing the config: if that vendor charges per call, do not let a framework cache be the only thing between you and the invoice. Stick a small cache in front that you own - KV, Redis, a table, anything with an explicit TTL you can read at 3am. Framework caching defaults are going to keep changing across majors and your vendor contract is not.

Put a counter on it. I patch global.fetch in a test that renders the route and assert the number of upstream calls per render is what I think it is. Took twenty minutes to write, has caught two of these, one of which was a dependency's SDK deciding to refresh a token on every request.

Billing regressions are the only regressions where nobody files a bug for six weeks.

revalidate only takes effect if you also set export const dynamic = 'force-static', otherwise it is ignored. That is why yours did nothing.

The build output legend is the most honest documentation this framework has and it is printed once, in grey, after 40 lines of nothing.