Exercise 1: A Third Caller of the Shared Function — Possible Solution
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THE NEW FUNCTION
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    def print_summary_email(order):
        total = calculate_order_total(order)
        return f"Order summary: your total was ${total:.2f}"

A third caller of this chapter's own extracted calculate_order_total,
alongside the existing print_invoice and print_receipt.

BEFORE THE FEE CHANGE
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    Total: $100.00, Tax: $8.00
    You paid: $100.00
    Order summary: your total was $100.00

AFTER APPLYING A FLAT $2 HANDLING FEE TO calculate_order_total ONLY
------------------------------
    Total: $102.00, Tax: $8.16
    You paid: $102.00
    Order summary: your total was $102.00

All three callers reflect the fee correctly, without any of them being
edited directly - including print_summary_email, which didn't even
exist when the shared function was first extracted.

WHY THIS WORKS AS AN ANSWER
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This is the chapter's own Extract Method finding, extended one caller
further: the guarantee that "editing the shared function fixes every
caller" doesn't weaken as more callers are added over time. A function
written today against calculate_order_total inherits every future fix
to that function automatically, the same as the two callers that
existed when the extraction was first performed.