postage costs through USPS
As far as I know, postage costs are based on:
Type of service used (express, priority, first class parcel, parcel)
Weight of packaged item
Volume (size) of packaged item
Distance to be sent
I don't think the value of the item INSIDE the package has a thing to do with the above calculations.
IF, however, one adds insurance and other services, then the value of the item inside would be relevant.