Programmatically persist the current values to initial values of environment variables.

Postman - Persist Environment Variables.js

    let headers = {'X-Api-Key' : ''};
    headers[`X-Api-Key`] = String(pm.variables.get("postman_api_key"));

    // Note: Update the environment UID if no longer sync
    const postman_api_url = "https://api.getpostman.com/environments/" + pm.environment.get("uid");

    let requestOptions = {
        url: postman_api_url,
        method: 'GET',
        header: headers
    }

    // retrieve first the complete set of current environment data
    pm.sendRequest(requestOptions, (error, response) => {
        if (error) {
            console.error("Error retrieving current environment data: " + error)
        } else {
            let body = response.json()

            console.log("Current Environment: " + JSON.stringify(body))

            // Update this env_keys array with the environment variable keys to update or create
            const env_keys = [
                'firstname', 
                'lastname', 
                'totalprice', 
                'depositpaid', 
                'checkin', 
                'checkout', 
                'additionalneeds', 
                'bookingid'];

            // start updating target environment variables
            env_keys.forEach((env_val) => {
                let index = body.environment.values.findIndex((item) => {
                    return (item.key === env_val)
                })
                if (index != -1) {
                    body.environment.values[index].value = String(pm.environment.get(env_val));
                } else {
                    let vars = {key: '', value: ''};
                    vars.key = String(env_val);
                    vars.value = String(pm.environment.get(env_val));
                    console.log("vars: " + JSON.stringify(vars));
                    body.environment.values.push(vars);
                }
            })
            console.log("New Environment Variables: " + JSON.stringify(body))

            requestOptions = {
                url: postman_api_url,
                method: 'PUT',
                header: headers,
                body: {
                    mode: 'raw',
                    raw: body
                }
            }

            //console.log("JSON.stringify(): " + JSON.stringify(requestOptions));
            pm.sendRequest(requestOptions, (error, response) => {
                if (error) {
                    console.error("Error updating current environment: " + error)
                }
            })
    }

Updated implementation using an array of environment keys code optimization.