Shipping can be Calculated or Flat Rate.. It sounds as if you should use the Flat Rate option for your International Shipping if you dont use UPS, USPS or Fedex. You can then enter the price for any country you ship to using the shipping company you use.
Also, our invoicing system allows you to change the shipping destination country for international or zip for domestic, you can also add or subtract to the invoice before sending it to the buyer.
Gazza