You could start at US Postal Service

On the left of the page is a selection for "Rates and Fees" that will open up choice to "Calculate International Rates" or to "International Rates," with the latter being the rate tables.

Note that there are competing systems. You should be able to find web sites for UPS (formerly United Parcel Service) or FedEx (formerly Federal Express), both of which, I'm told, do international shipments. Higher weights do run up the cost pretty rapidly with the USPS, so most stuff over a couple of pounds usually goes with one of the other services inside the US.

You own local package shippers may have "connections" with other shippers, and rates may differ when you pay at destination instead of at origin.(?)

John