The thermos Kim got Saul is his symbol of their relationship and the cap of the tequila bottle is her symbol of their relationship. It's what they see in each other.
Saul's is about Kim's thoughtful and compassionate sides, Kim's is about how Jimmy makes her feel alive and in charge of her life by going after actual bad people and in turn helping the little guy.
And there is a reason we see that Saul got their "relationship" (aka the thermos) shot by a bullet.
And to add to that, when they were pushing Saul's car into the ravine, right at the last moment, even in a state of shock, he had to stop and go into the car and grab the thermos. In Kim's last moment in the office, after telling her assistant to grab all of her things (like her diplomas, ect), she went into the desk herself and grabbed the cork b/c it's the only item that truly mattered to her. They both thought of nothing else during a high intensity moment in their different escapes.