I'm going to TRY to be brief.

I'm a seller, but also a buyer.  I know the costs of shipment and get frustrated by sellers who refuse to combine shipping or just completely overcharge all together.

As a seller, I know that eBay & PayPal take fees for the final costs.  As a way to avoid eBay fees, seller's sometimes pad the shipping - buyer's will never understand unless they sell themselves.  Seller's also sometimes add costs onto shipping for fees that buyers don't see, such as packaging (i.e. shipping peanuts - bubble mailers - bubble wrap - kraft paper - etc).

As a buyer, you can report a seller for excessive fees.  For example, if you purchase an item for 99 cents and shipping is $19.99, you know the seller is most likely guilty of some kind of eBay fraud and not paying fees.  eBay actually has a quick page for you to fill out for this kind of fraud.

These are the actual costs I've come across:

If an item weighs less than 13 ounces, it can be mailed 1st Class (as long as it's not huge).  First Class is $1.81-3.21 at the most (PA to CA).  When your choice is 1st Class or Proirity Mail - they both seem to take the same amount of time -- but Priority is $4.80 at the lowest cost.

Priority Mail is far better than Parcel Postage.  Priority takes much less time and is usually not much more than Parcel Postage.  When combining your purchases, sellers can often use a Flat Rate Priority Box that costs just under $10 with no additional costs to the seller. 

Parcel Postage is basically the lowest shipping you can get.  When I say lowest, I mean lowest.  When you live 1 state away from the shipper, if you choose Parcel Postage - it can take up to 3 weeks to receive.  This is rare, but I just experienced it!  A seller from New York shipped to me in Pennsylvania and it somehow was stuck in New Jersey for 2 weeks!  The Post Office couldn't explain it, they just said 'well you shipped Parcel, it can do that."  I've also shipped out twice and watched my packages sit through tracking at the same place for 2 weeks straight.  FRUSTRATING!

I, as a seller, especially for higher priced auctions will upgrade shipping for free.  Not an issue to pay the extra dollar or two. 

I do suggest paying for insurance if your seller offers it.  It guarantees that you get your item or your money.  If you're dealing with a bad seller, they're gauranteed to ship your item because you paid for insurance.  If they don't, they'll owe you money regardless.  If they don't offer insurance, they're still held responsible because they didn't offer it and have no proof that you received your item.

**Special Note** Make sure your address with PayPal is confirmed.  When a seller ships with PayPal you are guarenteed a tracking number so you can follow your item.  In this case, if the item doesn't reach you and you didn't purchase insurance...it is your fault.  You see that they shipped the item and a seller can never be held responsible for an item once it leaves their hands.

I hope this is helpful.  I just sort of blurted things out in no particular order.  I promise that my next review will be more thought out!