Payments
How to actually take money on the internet — Stripe (and friends), the difference between charges/subscriptions/Connect, idempotent webhooks, dunning, PCI scope, and the failure modes that cost the most.
How email authentication actually works — SPF, DKIM, DMARC, BIMI — plus the operational practices (warmup, bounce handling, unsubscribes, templates, transactional vs marketing separation) that decide whether your emails reach the inbox or the void.