Sounds like you bought a
Natal Plum
This isn't actually a plum of any kind, but an evergreen bush. *You can find more information in the
BCI Species Index by looking under the plant's latin name, which is Carissa.
This plant should not be exposed to frost. *Only water when the soil has lost some moisture and become just a bit dry. *Some plants will drop their leaves when exposed to stresses like changes in environment or cold in shipping.
You should be okay if you don't react by overwatering.
Regards,
Matt