You can ask your supplier for a lower cost for items in your contract, or to simply fix the current cost and avoid future fluctuation. This can be done by creating a Proposal for a Sub-contract from your supplier.
Instruction: go to your Contract app > View a contract > Create proposal, then wait for the response of supplier
Click Create proposal
Select the items to bargain > Create
Sub-contract (proposal) created
Wait for response from Supplier
