How to load resource for Lump sum contract in P-6.2

Hi,

My client instructed us to generate invoice through Primavera (Billing through Primavera P6) and my contract is 

a lumsump contract. The contract value given in the Cost Center part has a  list of items, the payment will be relased (by client) against the completion of the respective cost center items (to us).  We dont have the seperate BOQ, So What I did is,

1.  I just inroduce a seperate WBS as Cost Center in the master programme.

2.  added the each cost center items as activities under the new WBS.

3.  I linked the cost center items to the respective activities completion.

4.  Then I loaded the the cost  for the respective cost center items as budget units.

5.  Then I update the Physical progress & I generate the invoce.

Please any body advice the above methodology is correct or suggest any other methodology .

Reg,

Sri