Quoted from ForceFlow:

It seems like either way you look at it, the customer loses, either through restocking fees or increased price. Businesses generally don't want to "eat the cost", and with a smaller business, it's difficult to do that in the first place.
I get that for low value items, it's not that big of a hit. But when you start getting into the hundreds or thousands of dollars, it's a pretty significant sum.
What if you're selling a NIB game, and you get an accidental duplicate order? Whoops, that's at least a $150 loss or more. You can't really pass that along to the customer as a restocking fee. Plus, if you raise your prices to cover that, now you have higher prices than your competitors, which could lead to a decrease in sales.