The price of an international label is rarely the price of the shipment. Each destination sets a value threshold below which a parcel clears without duty or tax, and above it the buyer is billed on delivery. A parcel that arrives with a surprise bill gets refused, so the threshold matters as much as the postage. These 8 guides cover both.
I'd Ship That prints discounted USPS and UPS labels with the customs declaration built into the same flow, so the form is completed before the label is bought rather than at the counter.
Declare the real contents and the real value. A declaration that understates either is what turns a routine clearance into a held parcel.