Store shipping id inside checkout - if enabled in parameters and there is only one valid shipping, it can be directly stored
But then the cart needs to be reloaded to store the costs of the shipping and make ready for payment (payment gets info about shipping because of rules)
int$return 1 ... Association list, 2 ... Object list, 3 ... ID (be aware when setting 3, only first ID will be returned even more methods with the same method name can exist)