Grand Parkway & EXTagXPRESS?

Hello,

I’m coming from Vidor to Spring area tomorrow. I’ll be going during peak traffic time, and I’m really wanting to avoid heavy traffic. I want to take SH99/Grand Pkwy to get there but I don’t have an EZTAG and it doesn’t seem like I can buy one anywhere near me. I just downloaded the EZTagXpress app and I was wanting to know if I can ride through 99 and just pay on the app? Is this risky? Could I get a ticket doing this or get pulled over if a cop sees me without a tag?

edit: typo in the title. I meant to type EZTagXpress.