I assume that whatever class your ProductsComboBox contains has overridden ToString, to give you the human readable value. If so, then just cast the object to your class:
int ProductIndex = ProductsComboBox.SelectedIndex;
MyClass selected = ProductsComboBox.Items[ProductIndex] as MyClass;
if (selected != null)
{
string productName = selected.ProductName;
...
}
Or better:
MyClass selected = ProductsComboBox.SelectedItem as MyClass;
if (selected != null)
{
string productName = selected.ProductName;
...
}
Presumably, your class contains the ID already!