If you don't know why people charge more for shipping, you haven't sold much on Ebay. Sometimes you can't help it. Other sellers set the item price and shipping so a bidder will only be willing to pay so much for an item.
If the going rate of that cable modem is $51 and everyone else is charging $15 for shipping, when it actually costs $10 to ship, you're losing $5. If you wanted $56 for the modem and $10 shipping when everyone else is getting $50 and $15 for shipping, your item will not sell. Most STUPID buyers just look at the auction price and not the shipping. My point is that it's not only the sellers fault, but it's the buyers fault as well.
Also.. People post fixed shipping and handling charges out of convenience. With zone shipping, it may cost $7 to ship it locally, but it may cost $12 or more to ship it to California from the East Coast. You don't know who you are selling to and where they are from until they win your item. You could charge actual shipping to be figured out after the auction, but that takes extra time. Most people want to win the item then click on the PayPal link, pay for the item and have it shipped ASAP! It might eat up another day and several emails to find out actual shipping charges. Then again, who knows that this person would even be looking at your auction because you'd be charging $5 more for your item because you are charging actual shipping charges.
If you don't like someone's shipping charge, don't bid.
Sal