Picture of the tree.

Your plant actually looks really healthy. Citrus likes a lot of sunlight and hate wet feet. Make sure you let the roots dry out a little before watering. You should check the drainage holes on the bottom of your container before watering. Feel the potting soil through the drainage holes and if it is still wet or very moist, you may have been overwatering and possibly causing leaf drop. The leaf drop may have also been caused by the stresses of shipping.

Members of this citrus forum highly recommend the Gritty mix or 5-1-1 mix for citrus, I forgot which one. The potting soil should have very good drainage. Commercial potting soils often contain too much organic matter which holds excessive amounts of moisture which eventually causes root rot and death of the plant.
Simon