Hi,
As of my knowledge, there is no option in standard (ECC) to link the service contract and normal material PO freight condition type. Instead, you can create access sequence for the condition record and maintain the agreed price with validity date. But if you want to control the total price based on contract, there is no option in standard as of my knowledge.
If you can think about Transportation Management (LE-TM), then you may create a service PO with validity period and system can always create SES against this PO from shipment cost document. The freight value can be maintained at TM condition record level (TK11) and the same can be loaded to material cost at the time of GR. It will ensure the three way match as well. But, inbound delivery is mandatory in this case. Refer the note: 427944 - Info: Shipment costs as delivery costs during goods receipt
Regards,
AKPT