Napa may pay more for the part because they buy less or because they couldn't get the same deal as other companies for any reason. Other shops may sell more alternators in order to charge less and profit more. There's also the usual supply and demand issue to consider. In stores, all costs are multi-dimensional. I recently lost the keys of my car, and I had no duplicate. Instead of changing the lock, I went for a locksmith service, and they made a few pairs of keys for me, so I will have a reserve even if I lose one pair.