The closest station to Costco is Kaihin-Makuhari station. Even from Kaihin-Makuhari station it is listed as 20 minutes walk, approximately 2 km or 1.2 miles in distance.
Also, there is no direct train service from Narita Airport to Kaihin-Makuhari. Using JR line you need to transfer twice to get from Narita Airport to Kaihin-Makuhari. Route will be:
Narita Airport – Chiba – Soga – Kaihin-Makuhari
Including transfer time it will take about 1 hour and 50 munities one way, with fare of JPY 950.
My question will be are you that set on going to Costco or are you open to other options?