I am an avid eBay buyer and have come across lots of sellers who rip you off for shipping.  Hopefully people will see this guide.  I am sure there are more like this out there, but my own buyer base (which I care about them a lot) could reach this through me and may not see another one.  I have broken this down into an International category, a Canada category, and a USA category, since the rates differ and in my experience so do the combined shipping cost general rules and things.  My area of expertise is with clothing, so that is what I will write about.  electronics and other things that require more careful packaging may cost more to package and mail with special care and materials, so I am only making claims about CLOTHING and similar items (perhaps linens or plush animals or something soft and easy to package?) and my experience shipping them.  Please keep that in mind when reading this.

THE FLAT RATE ENVELOPE:
I have recently discovered how great these are.  It is USPS Priority Mail (the fast service) at the lowest possible rate for anywhere in the world!  They are about the dimensions of a sheet of standard printing paper (maybe a touch larger), but they can fit WONDERS into them.  I can fit a T-Shirt (sometimes 2) or a mini skirt or sometimes even a pair of (plus sized) jeans or a non-poofy dress into one of these!  They are GREAT for single purchases!  In the USA, including cost of delivery confirmation, they cost $5.45 to mail anywhere in the country!  Canada/Mexico is $9.95, and International is $11.95 for anywhere in the world!  The international rate is great compared to what I have seen some people charge to mail the same kinds of items that can fit into one of these babies!

THE FLAT RATE BOX:
This is another great gift from the USPS mailbox in the sky.  A box that you can shove as much stuff as possible in it and mail A-NY-WHERE for one single rate!  In the USA, a flat rate box, standard size (fits 2-7 items depending what they are) costs $10.45 including delivery confirmation ($9.80 without).  Canada/Mexico same box costs $23.95, and internationally it costs $38.95.  Unless you are ordering something HUGE that requires special packaging or a larger flat rate box ($12.95 USA, $29.95 Canada/Mexico, $49.95 Internationally), be suspicious if charged more than these rates.  I can fit 2 poofy dresses or 4 pairs of jeans or 7 T-shirts (or various assortments/quantities/combinations of these items) into one of the small size and have NEVER had to use a large one, so keep that in mind.

USA
For combined purchases, it SHOULD NOT cost much extra per item after the heaviest item, MAYBE $2, but not much more unless it's another heavy item (in which case they should look into a flat rate box).  If they charge you more than $10.45 for combined clothing (2-5 items average), they are 99% likely RIPPING YOU OFF. $10.45 is the exact cost of a USPS Priority Mail Flat Rate box with delivery confirmation.  You can fit a lot of stuff in there if you fold and shove.  Any seller who ships Priority and charges you for labor + packing material is lying, since USPS gives free supplies AND offers free pickup of your pre-paid printed labels!  All they really have to do is  put the item in the box.  If it's $6 for a T-Shirt shipping, make sure that is for Priority.  First Class cost $3 to ship a T-shirt.  Oh yeah, and if a seller lives within a few states of you, let them know and ask for a discount.  Shipping is based on weight as well as location (unless it's flat rate), so if they are doing regular shipping, no flat rate, you should get a cheaper price.  Most sellers price their domestic shipping as if they have to ship the farthest possible location (I do this to cover my costs if someone from the West Coast, Alaska, or Hawaii wins, but I give a discount to people who are within my general area).

CANADA/MEXICO:
If you are buying from a US seller, you should DEFINITELY have a discount in shipping.  A USPS Priority Flat Rate Envelope costs $9.95 (as opposed to $11.95 worldwide), and a USPS Priority Mail standard flat rate box costs $23.95 (as opposed to $38.95 worldwide).  Normally to mail a poofy dress to Canada in a USPS Priority Mail  box (weight-based, not flat rate), it costs me $18.40 (as opposed to $25-27 worldwide).  If a seller is shipping First Class, the rates will be almost identical to USA rates.  I have mailed a shirt to Canada for $4 including the cost of the padded envelope before.  Canada/Mexico buyers, if you are buying from USA, you are entitled to a discount, and if the seller will not give you one, they are taking advantage of you.

INTERNATIONAL:
International shipping rates are one of the worst almost-scams I've seen!  One buyer complimented me on my rates because she told me that some sellers wanted $20-30 just to ship a T-Shirt to Europe!  For T-Shirts and mini skirts and smaller items, a USPS Priority Flat Rate envelope costs $11.95 to mail ANYWHERE IN THE WORLD, and it's the fast service too!  First Class (or Parcel Post, depending on weight) is a good option for heavier items (such as pants that won't fit into the flat rate envelope and things that can be folded flat but are heavier).  It really isn't worth it for a T-Shirt or other lightweight item, since it would cost around $10 for the slow service and Priority is only $2 more and faster.  Priority Mail boxes start around $22 as an international rate, so for a coat or poofy dress or other item that needs a little more room and should not be squished into an envelope, it actually does cost that much to mail it.

ALL BUYERS:
If you feel that a rate is a bit high, ask for a discount and/or ask the seller why does it cost so much, and if they refuse or give you a roundabout crappy answer and end up ripping you off (check the labels and make sure first), just leave a neutral comment.  Praise the item (assuming it is good) but complain about their shipping charges, it won't hurt the score but won't raise it either and keeps other buyers informed and lets them know to keep shopping around before buying.  And be sure to give low star rating for the shipping charges.  Those count too, as they determine Power Sellers' fee discounts and all sellers' search visibility when searching with BEST MATCH.  But don't give low star ratings on Communication, Accurate Item, or Shipping Time unless there was a problem with one those specifically in addition to shipping charges.  It only takes one low star rating to mess up a seller's status with discounts and visibility, so be honest when rating their performance in the respective categories.

When I contact sellers about shipping, it's around 50/50 what I get.  sometimes I get one who will talk in circles and flat-out lie to me or will refuse to work with me, sometimes I get a great cooperative seller who helps me out lots.  If you're not in a hurry, ask for a slower service and shave a few dollars off the cost.  If you want Priority, know your flat rate envelope prices (for single purchases) and be sure to ask about them.  One thing I have found effective in getting a good rate (especially for combined) is that once you win the additional item(s), IMMEDIATELY send a message to the seller before they have time to send the invoice and show your knowledge!  Say something like "I won these things from you and would like combined shipping.  You could put them in a flat rate box for $10.45 including delivery confirmation."  Or if it's something like 2-3 shirts be like "I know it shouldn't cost any more than $8-9 to mail these things Priority Mail..."  USE YOUR KNOWLEDGE!  A seller is less likely to rip you off if you know what it costs!

Hope this helps somebody!!