I have two pending transactions that are scheduled to close after August 13. I am representing the buyers in both transactions and each listing had an offering of compensation to a cooperating broker published on the MLS at the time that I wrote my offer. Do I need any special addendums or agreements completed before closing to be sure that I am paid?

Additional documentation is not required in these cases as you are already protected. Any agreements written but not closed prior to the date of the NAR policy change must still honor that commitment. This information is reiterated in the NAR FAQ #36 which reads as follows:  

Can buyers and buyer brokers rely on an offer of compensation that was on an MLS prior to the effective date of the MLS policy changes? If the sales contract is executed before the MLS policy change, the buyer broker should be able to rely upon the offer of compensation even if closing occurs after the date of the policy change. But if a sales contract is not executed before the date the participant’s MLS implements the policy changes, the offer on an MLS will not be valid and buyers and buyer brokers may wish to protect themselves in writing with the listing broker or seller through a broker agreement or by including the offer of compensation in the sales contract.