For your concern about changing the shipping method on Suppliers, please help me to follow this instruction:
Go to Ali Orders > Settings > Suppliers > Switch it into shipping method you want > Save settings.
By this setting, the selected shipping method is applied as default for all your orders when you fulfill orders.
OR you can simply change the shipping method for a specific order when you are fulfilling your order.
When you are doing your fulfill order > 2. Review and confirm your order > Right next to the price of products ~> You can simply switch it into another shipping method as the photo below has showed.
I hope this might be helpful for you.