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.

That's correct. On international packages, you do have to put the value of the contents on the customs form. But that doesn't affect your mailing cost.

I never put costs over $15.00 because they would end up paying duty charges.

check against postal rates .....flat rate boxes are not always cheaper ....

Priority flat rates for international are cost more than the rate for sending domestic. When I recently sent a package overseas I found out that any package (not necessarily an envelope under 3/4 inches) under 4 pounds is one rate no matter if it is just under 4 lbs or 8 oz. and if it is over 4 its even it is higher. I think Canada is a little cheaper than going overseas.

Go to the USPS website for explanations and steps to compare different types of shipments.

Tell me about it!! My DD, hubby & my 5 DGK live in China. When sent Christmas pkgs it nearly breaks me. One time postage was $91, $85, etc! Anything is expensive.... Oh, last Dec. I send Christmas gifts & it took 3 wks to get there. One got there in 1 wk. The other got lost, checked into it, good news: they found it; bad news it was in Mexico. So that pkg went from Texas to Chicago, to Mexico & finally to China!! So they got belated Christmas gifts.... But only way to ship. I has asked what ever happened to "that slow boat to China?"...