2way + 2way or 2way + 3way?

Wages of full-time and most part-time employees, unless hours are very flexible, are not considered marginal cost.  In any event, there are fixed costs and quasi-fixed costs associated with all employees. 

Online retailers win because of lower FIXED costs AND marginal costs. 

Fixed:  lower wages, rent, utilities, etc.  (normal costs of staying in business not normally directly proportional to the cost of another item sold or produced)

Marginal:  additional cost of producing or selling one more unit of output.  This would be the cost to process one order, shipping, materials, cost of goods sold, commission, etc.

Because of BOTH marginal and fixed costs being lower for online retailers they can lower profit margins and make up for the lost margins in volume of sales.

Be weary of online retailers.  Few are authorized and you may be getting refurbished or b-stock goods.  Few offer real warranties.  This surely reduces their cost.  :-)