When shopping online I think it all comes down to total price including shipping and the convenience of shopping from home. How much gas would I burn driving around town or maybe even to the next town to find exactly what I was looking for when I know I could find it online at a reasonable TOTAL price at midnight on a holiday in the pouring down rain with no make-up on.
Keep in mind that most online shops have to pay a percentage of the sales price not including shipping to the host site (Ebay, Etsy, Yahoo, Amazon, and even their own website hosted by another party) They still need to make a profit off the sale so they lower the price a little to bring down the percentage they have to pay, and add it to shipping. Shipping is not free to the seller, in fact it is way more than the actual postage as Patrice said. Lots of other costs involved.
TOTAL PRICE and convenience is the bottom line.