It's only possible to define shipping destination countries for your entire account, not on a product-specific basis.
To set a different flat-rate for a specific product, you would pick a specific packaging container type that would only be used for that product, set its Cost to your preferred flat-rate for that product. Then just configure the product to ship in that packaging container type and set its packing capacity to how many units of the product can be shipped for that rate (e.g., if your flat rate is per-unit, set capacity to 1, for up to 5 units set capacity to 5, for any number of units set capacity to some absurdly high number like 9999).
