One of the most popular email services, Gmail keeps track of all your emails with threaded conversations, tags, and Google-powered search to find any message you need.
Send approved supplier agreements for signature and update the buyer.
Your signer gets a private link to sign the agreement. Your coworker gets a separate link to view the request.
Send supplier agreements to the required signers.
Your final agreements reach the signers you list, and you get a reply with the invitation details in the original email thread.