The pro labs around my way used to provide additional services to their professional customers, and offer them lower prices.
In order to get the benefit of those services and prices, I used to have to supply a business licence, a tax number and some evidence of both a photographic business and some competence.
The additional services included a variety of masks that permitted a photographer to crop negatives but still obtain high quality, low cost machine prints with individual colour correction.
The lower prices worked for the labs because professional photographers tended to have higher volumes, and if the photographers were competent, the labs could put high quality work through more efficiently.
Unless I was able to qualify for a professional account, I wasn't able to get access to the special professional list of services/price list.
That business model is long gone, but I expect Millers just hasn't changed their system.