Need help from costing experts

Hi experts,

we are implementing navision for manufacturing company.They have items
1. Raw material
2. Stores& spares(for machine repair purpose)
3.electrical parts(machine repair purpose)
4.Finished goods(After raw material convert in to finished)
Client have above 4 items. Here my problem is which costing method is applicable to raw material, which costing method is applicable to finished goods, like remaining.
Presently they are not unsing any costing method. Right now they selecting randomly for issue material,for issue stores & spares, for any item they are using random method. They are not following any particular method.
Is there any rule t o finished product should have particular costing method ?
Is there any rule to particular item must have particular costing method? I read costing method . I know little bit functionality of costing method.But iam getting confuse which costing method is applicable for each item
CLIENT ASSIGNED THAT COSTING METHOD SELECTION TO US.SO WE HAVE TO TAKE CARE.

really its killing me. EXPERTS PLEASE PROVIDE SOLUTION.
Experts u implemented a lot of projects.
U had used costing methods.So give u r valuable suggestion to me. Please provide solution.

thanks in advance.