Restoring previous session from id?

I'm stumped on something.

I have a checkout process where a user enters billing info and selects a product, both of which are stored to the session (not written to the DB). One of my payment methods is Affirm, which generates a checkout link that I forward the user to, and on success, affirm sends a post request to my callback route. Problem is, on that callback route I can't restore the session to finalize the transaction (e.g. write the model to the DB and fire events).

I can send metadata along with the payload of the payment processor to return to my callback, so I put the session id in there, but on the callback method I can't hydrate the session from that id. I have the user id but logging in with that just generats a new session key and I lose my billing data and model in the session.

    // user is logged in and adding trip to cart
    public function addToCart(Request $request, Trip $trip)
    {
        session()->put('cart_trip', $trip);
        return view('some-view-here');
     }

Upon checkout, I redirect them to Affirm's application. Upon success, they send a post request to me with a given payload. In this instance, I added the session key.

    // post request handler.
    public function affirmSuccessCallback(Request $request)
    {
        $charge = Payment::authorizeAffirm($request->input('checkout_token'));
        session()->setId($charge['details']['metadata']['session_id']);
        
        // this is null
        dd(session('cart_trip'));

        // can't do this either since it'll just make a new session
        Auth::login($charge['details']['metadata']['session_id']);
    }

I need a way to restore that session to access the model and billing data that was filled out during the checkout process on my site before going to Affirm.. ideas?