Find the answer to your question
How can I provide shipping costs for combined payments for both domestic and international shipping?
What you can do is to set your Shipping Discounts in your MyeBay Shipping and discounts selling preferences. You can select the
"Charge my highest shipping cost for the first item and US $" and set the value to 1.50
If a buyer goes into checkout, and has a US shipping address and a combined order for the 3 purchases comes up, the "Shipping and handling" drop down showing the shipping services will automatically take into consideration that each item after the first is 1.50 ... here is an example:
Let's say you provide USPS Priority with a cost of 5, USPS First Class with a cost of 6, and USPS Express Mail with a cost of 7, and the buyer purchases 3 of these items, the eBay Review Your Purchases page will show these 3 shipping services in the drop down with costs of $8 (5 + 2x1.50), $9 ($6 + 2x1.50), and $10 ($7 + 2x1.50). The discount setting you have is automatically calculated into the shipping cost shown.
Unfortunately, the Shipping Discounts settings does not allow for a different discount for international shipping, so you will have to use an explicit AddOrder call for customers that have an international shipping address. In the explicit AddOrder call you would indicate the international shipping services you are providing at the Order level, and the total shippingservicecost for the combined purchase for each shipping service. Let's say you had 3 international shipping services with the same costs as in the previous example, $5, $6 and $7, then you would make the shippingservicecost for each of these shipping services at the Order level in AddOrder as $9 ($5 + 2x2.00), $10 ($6 + 2x2.00), and $11 ($7 + 2x2.00). If you wish you can also break down each cost at each transaction in the AddOrder call, but it's not necessary since what takes effect is the data at the Order level used in the AddOrder call.