None of the billing types will let them add stock items to the Service Contract in the Services per Period tab. (because it's for services, or non-stock items).
You have 2 options:

1. End Period Plus service contract: Add a service to with the amount they want to bill per period. Include the sotck items (and other items they need) to schedules. Generate the appointments or service orders from the schedules.
   They can also create the service orders and appointments manually and link them to the service contract. 
   Select the Free Item checkbox for all materials included in the contract price.
   Generate one invoice from Run Service Contract Billing at the end of each period (also all the related appointments need to be closed)
2. Beginning Period Fixed : Add a service to with the amount they want to bill per period. 
   Include the sotck items (and other items they need) to schedules. Generate the appointments or service orders from the schedules.
   They can also create the service orders and appointments manually and link them to the service contract. In this case, all will be marked by default as Free Item. So they would have to unselect all the materials they want to bill. 
   Generate two invoices (at least): one invoice from Run Service Contract Billing, that will have the service added to the contract.
   Another invoice created directly from the appointments (or service orders, depending on the customer's billing cycle) with the additional (services or stock items).
   
   Thanks!