Challenge 1: Polling vs Webhook — Possible Solution
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RECOMMEND: Webhook.

JUSTIFICATION
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The whole point of this scenario is that the customer should be
notified the INSTANT the driver marks the order delivered — timing
matters directly to the user experience here. Polling would mean the
app repeatedly asks "is it delivered yet?" on some interval (say,
every 10 seconds), which means:
  - Wasted requests during all the time nothing has changed yet
    (most of the delivery window).
  - A real, noticeable delay between the actual delivery event and
    the customer finding out — up to the length of the polling
    interval, which directly undermines the "notify immediately" goal.

A webhook fixes both problems at once: the delivery-tracking system
pushes a notification the moment the driver marks it delivered, with
no polling overhead and effectively no delay between the event and
the notification going out.

WHY THIS WORKS AS AN ANSWER
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This is exactly the situation webhooks were designed for, per the
chapter's opening comparison: an event that happens at an
unpredictable time, where the DELAY between the event and finding out
about it has a real, direct cost (a worse customer experience). When
immediacy genuinely matters and the event is infrequent/unpredictable
rather than constant, a webhook is the better fit over polling.