Buying the comprehensive package when you needed a refraction
If you are young, healthy and simply need updated glasses, the full screening tier is usually money spent on reassurance rather than need. Match scope to situation: screening earns its price for over-60s, diabetics, and anyone with a glaucoma family history — not for every routine prescription top-up.
Accepting a verbal prescription or a 'we'll make your glasses' promise
Without a complete written prescription including pupillary distance, you are captive to the tester's own glasses pricing. The written document is what makes the market work for you — any optical shop can fill it. No paper, no payment.
Waiting for a child to complain before testing
Children rarely report blurry vision because they assume everyone sees the way they do; the signals are squinting, sitting close to screens, and slipping schoolwork. By the time a child complains, myopia may have progressed for a year or more. An annual test through the school years is cheap insurance against exactly this.