A lot of people overcharge S&H costs. I would like to think most of those people simply do not know the post office rates, so they make an educated guess. Some others may undercharge.

Here is a little bit about S&H and the proper cost as of April 08'

Express Mail

This money-back guaranteed overnight to two-day service includes tracking, proof of delivery, and insurance up to $100. Additional insurance up to $5,000 may be purchased for merchandise. All packages must use an Express Mail label. The maximum weight is 70 pounds. Sunday and Holiday delivery available to many destinations for an additional fee of $12.50.

When sealing a flat-rate box or flat-rate envelope, the container flaps must be able to close within the normal folds. Tape may be applied to the flaps and seams to reinforce the container, provided the design of the container is not enlarged by opening the sides and the container is not reconstructed in any way.

Features
Express Mail envelopes, labels, and boxes may be ordered online at The Postal Store and are available at Post Offices. Features include delivery to Post Office boxes and rural addresses, and domestic rates for APO and FPO addresses.

Express Mail delivery status is available with Track and Confirm.

Express Mail Rates for Post Office to Addressee Service
Weight
 Rate*
 
Up to 8 ounces
 $16.25
 
Over 8 ounces up to 1 pound
 19.50
 
Over 1 pound up to 2 pounds
 21.40
 
Up to 3 pounds
 24.50
 
Up to 4 pounds
 27.60
 
Up to 5 pounds
 30.70
 
Up to 6 pounds
 33.80
 
Up to 7 pounds
 36.90
 
Up to 8 pounds
 40.00
 
Up to 9 pounds
 43.10
 
Up to 10 pounds
 45.25
 

*Sunday and Holiday delivery available to many destinations for an additional fee of $12.50.

Priority Mail

Priority Mail is used for documents, gifts, merchandise, and any mailable item weighing 70 pounds or less. The maximum size is 108 inches or less in combined length and distance around the thickest part. Mark each package "Priority Mail" in the postage area or use a USPS-provided Priority Mail envelope or box.

When sealing a flat-rate box or flat-rate envelope, the container flaps must be able to close within the normal folds. Tape may be applied to the flaps and seams to reinforce the container, provided the design of the container is not enlarged by opening the sides and the container is not reconstructed in any way.

Features
Priority Mail service offers two- to three-day service to most domestic destinations. Priority Mail envelopes and boxes are available at many Post Offices and can be ordered online at The Postal Store. You can even print your own Priority Mail labels and postage with Click-N-Ship.

Priority Mail "Balloon Rate"
Parcels that are addressed for delivery to zones 1-4 and weigh less than 20 pounds but measure more than 84 inches in combined length and distance around the thickest part must pay the 20-pound Priority Mail zoned rate.

Priority Mail "Dimensional Weight"
Parcels addressed for delivery to zones 5–8 that exceed 1 cubic foot (1,728 cubic inches) are charged based on the actual weight or the dimensional weight, whichever is greater.

Priority Mail Rates Weight
not over ___ (pounds)
 Local Zones (Listed in order down)
1 & 2,  Zone 3,  Zone 4,  Zone 5,  Zone 6,  Zone 7,  Zone 8 = 

1lb.
 $4.60
 $4.60
 $4.60
 $4.60
 $4.60
 $4.60
 $4.60
 
2
 4.60
 4.90
 5.30
 6.20
 6.55
 7.00
 7.50
 
3
 5.05
 5.70
 6.40
 8.25
 9.10
 9.65
 10.55
 
4
 5.70
 6.65
 7.70
 10.20
 11.10
 12.20
 13.45
 
5
 6.30
 7.55
 8.90
 11.90
 12.90
 14.35
 15.85
 

Parcel Post

Used for gifts and general merchandise, Parcel Post shipments may also contain books and other printed matter weighing 70 pounds or less. The maximum size is 130 inches in combined length and distance around the thickest part.

Parcel Post "Balloon Rate"
Items that weigh less than 20 pounds and measure more than 84 inches but not more than 108 inches in combined length and girth (distance around the thickest part) pay the 20-pound Parcel Post® zoned rate.

Parcel Post Oversized Rate
Items that measure more than 108 inches but not more than 130 inches in combined length and girth (distance around the thickest part) pay the oversized rate regardless of weight.

Parcel Post Nonmachinable Surcharge
A nonmachinable surcharge applies to certain items of unusual weight, size, or shape.

Priority Mail; Large Flat-Rate Box

When sealing a flat-rate box or flat-rate envelope, the container flaps must be able to close within the normal folds. Tape may be applied to the flaps and seams to reinforce the container, provided the design of the container is not enlarged by opening the sides and the container is not reconstructed in any way.

Sizes: 12 inches long, 12 inches long and 6 inches thick

Price - Domestic:

Domestic Military APO/FPO destination ZIP Codes: $10.95
All other destination ZIP Codes: $12.95
Regardless of weight for items mailed in the Priority Mail Large Flat Rate Boxes provided by the Postal Service.

Price - International:

Canada and Mexico: $29.95
All other international destinations: $49.95
Online 5% discount available through Click-N-Ship to international destinations. Weight limit is 20 pounds to international destinations for items mailed in the Priority Mail Large Flat Rate Boxes provided by the Postal Service.

APO/FPO Priority Mail Large Flat Rate Box

The APO/FPO box can be identified buy the two markings, "APO/FPO Flat Rate Box" and "Americasupportsyou.mil." The price for this box to APO/FPO destination addresses is $10.95 or $12.95 when sent to domestic addresses. This box can also be used for international destinations.

General Priority Mail Large Flat Rate Box

The box can be identified buy the marking, "Large Flat Rate Box." The price for the box to non APO/FPO destination addresses is $12.95 or $10.95 when sent to APO/FPO addresses. This box can also be used for international destinations.

Priority Mail; Small Flat-Rate Box

When sealing a flat-rate box or flat-rate envelope, the container flaps must be able to close within the normal folds. Tape may be applied to the flaps and seams to reinforce the container, provided the design of the container is not enlarged by opening the sides and the container is not reconstructed in any way.

Boxes available in two sizes:

Box One: 11-7/8 x 3-3/8 x 13-5/8 inches (ideal for garments, board games, books and other relatively thin items).
Box Two: 11 x 8-1/2 x 5-1/2 inches (perfect for shoes, model cars, and taller items).

Rate is $8.95, regardless of weight or destination for items mailed in the Priority Mail Flat-Rate Boxes provided by the Postal Service.

Media Mail

Media Mail is used for books, film, manuscripts, printed music, printed test materials, sound recordings, play scripts, printed educational charts, loose-leaf pages and binders consisting of medical information, videotapes, and computer-recorded media like CDs and diskettes. Media Mail cannot contain advertising.

The maximum size is 108 inches in combined length and distance around the thickest part.

Mark each package "Media Mail" in the postage area.

Media Mail Rates Weight Not Over ____ (pounds)
 Rates
 
1lb. - $2.13
 
2 - 2.47
 
3 - 2.81
 
4 - 3.15
 
5 - 3.49
 
6 - 3.83
 
7 - 4.17
 
8 - 4.51
 
9 - 4.85
 
10 - 5.19
 

I hope you will find this guide helpful, if you do then please rate a "Yes"!