That sentence is using tenían because the subject is a "neutral they", it is a they referring to the people who serve in the shop. It's very common to use the plural in this cases. Other examples could be:
En Amazon me han dicho que no pueden enviarme el paquete urgente.
In Amazon they've told me they cannot send the parcel urgently.
En esta pastelería venden unos pasteles muy ricos.
In this bakery they sell very tasty cakes.
I hope this clarified it.
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