Update: the preview now shows the text a failed rule expected and what it actually found. In the synthetic EUR receipt, the result points to #total and shows €24.00 vs €29.00. The USD and discount variants can be checked separately on the scenario page.

One useful habit, even if you already use a test runner: take the expected total from a known fixture or business rule, then render the email and compare the selected text. If the same formatter generates both values, a formatting bug can pass both sides.

If you are changing a receipt or invitation right now, I would be interested in one rule from that change: what do you check today, and would the free preview linked above replace any step? A dummy example or a description is enough.