Cautionary tale: Opticians

I went for an eyetest. A woman with 3 years post-grad training sat down for 45 minutes, ran a battery of tests, detected a bacterial infection in my eye, sent me for a hospital appointment the next day, and gave me a prescription that I filled out online. My payment to her - 25 quid.

I did get her some chocolates to say thank you.

Now name another (para-)medical area where the medical part is a loss leader for the rest? (I am worried i will hear a lot of Americans ...)