If you are setting up the delivery option for Subscriptions & Preorder, then you must ensure that the service level matches the default service level for that delivery zone.
Here is an example set up -
On the image above you can see that Subscriptions has it's own Delivery Option called 'Standard_sub' but with a service level of 'Express'.
When you view that Delivery option and click 'Fulfilment Routes' you can see that it has been set up using the correct, Standard, shipping methods (click here on creating delivery options & fulfiment routes for a zone)
Of course this does mean that the Standard_sub delivery option cannot be used in Telesales, which is why you will see on the image above that there is a separate 'Standard' option which is just allowed for Telesales and has the service level of 'Standard'.