There should be a pdf version of your new registration after you purchase the renewal online, you can either download that, or take a screenshot when viewing it. I've used it once when pulled over and the cop was fine with it. It's literally a digital copy of your registration.

Hi Jessika,

Once your registration is legally renewed, eTags sends a confirmation email with a digital copy of your new registration to use while await delivery of your hard copy. If you have any additional questions, please give us a call at 888-633-5332. One of our friendly agents will be happy to assist you.

Kind regards,