What to call someone who buys only certain products (e.g. vegetables) from a certain store?

The shopper could be "habitual" to further imply they are regular and predictable in their purchases.

dictionary.com

adjective

of the nature of a habit; fixed by or resulting from habit: habitual courtesy.

being such by habit: a habitual gossip.

commonly used, followed, observed, etc., as by a particular person; customary: She took her habitual place at the table.

I'd like to place emphasis on the fact that he/she buys only a certain type of products from this store.

She is product-selective when she shops there

or

She is product-directed when she shops there

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EDIT

I originally included the following possibilty

She is product-biased when she shops there

However user405662 comments "selectiveness need not imply bias."

I could appeal to the following definition

the fact of preferring a particular subject or thing:

She showed a scientific bias at an early age.

https://dictionary.cambridge.org/dictionary/english/bias

... but the primary definition of bias usually refers to "unfair" preference. What unfairness signifies with regard to shopping is debatable, but I will concede the point.

choosy

difficult to please because you are very exact about what you like:

Cambridge

How about:

invariable: not changing or capable of change : CONSTANT (Websters)

or

inveterate: firmly established by long persistence. (Websters)