You cannot negotiate with the brand after it has selected you and the contract has been executed.
When you applied to the application you either agreed to a set rate or you shared your requested rate. If the brand selects you at either of these rates, the contract is executed and you are on the hook for everything in the contract including the number of pieces of content, the posting deadline (if applicable), and content direction specifics.
Please only apply to campaigns that you truly want to participate in (at the proposed rate or your suggested rate) and have the bandwidth to complete on time.