Ordered something for $4, shipper shipped with tracking using USPS, how is it possible?

Ordered $4 item on ebay (not printed matter or book), what kind of pricing do these sellers get that they are able to ship using USPS Ground Advantage that they are able to survive. I am always confused how this works, and what special pricing is available to large sellers, if any. Shipped from US, and to US, arrived in 3 days.