Upload & Sell: On
It depends in part on the language in your contract. Cancellation is a change to the contract. If your contract states that the contract may only be modified in writing, signed by both parties, then relying on a simple email is risky.
PDF the email, and write a one-paragraph AMENDMENT TO THE AGREEMENT, stating that the agreement is cancelled and explaining how any fees will be handled. For example, "Per CLIENT'S email dated xx/xx/xxxx, attached to this amendment, the AGREEMENT (also attached) is cancelled, and photographer is released from all obligations contained therein. CLIENT'S retainer of $xxx is retained by PHOTOGRAPHER, and applicable to a photography session completed prior to xx/xx/xxxx."