Hi,
Let's say your requirement is to Transport Steel and you want to have Yearly contract with a Freight vendor for that.
=> You create Service Master Record in AC03 using the right UOM like KM
=> Create Value Contract (WK) in ME31K with Item Category D and relevant AAC.
In that case you need not to use Freight condition in PO again. Let's look at your questions now;
1) Each time you create PO (Release Order) with reference to the Contract and Vendor receives the same. While invoicing he refers the same PO number
2) Go to ME3L / ME3M / ME3N and use Item category D as the reference parameter and you will get the Service Contracts and you can know whether it exists
3) In the above said T codes you can see what % of the contract value already consumed and thereby what is left to be used.
Thanks
NR