Hello Kasi,
Many thanks for your contacting us. I am Vincent, working as Senior Account Manager and Consultant at Cmsmart. My pleasure to support you in this case.
Is this theme compatible with Magento2.3.1?
=> Currently, our theme is compatible with magento 2..3.0.

Can I use this theme life time if i purchase with one time standard payment?
=> We provided 2 types of payments: Auto-renewal and Standard Payment.
** Auto-renewal: You will pay a fixed amount first ($79.79) for the payment to get support, download and upgrade within 12 months. After 12 months, you just need to pay a small fee (for every 12 months) $30.36 to extend more 12 months of support, download and upgrade (called renewal fee). Of course, if you do not like to pay the renewal fee, you can cancel it at any time. In this case (if you cancel it), you still can use our product for a lifetime but without our support, download and upgrade.
** Standard Payment: This is a one-time payment $85.38. You’ll pay one fixed payment to get our product. You also get our get support, download and upgrade within 12 months. After 12 months, if you want to extend your support, download, and upgrade, you have to pay 55% of the first price to renew your support, download and upgrade. In case, you do not renew it, you still use our product for a lifetime but without our support, download and upgrade.
Based on my experience, you should choose the auto-renewal because you can renew product at very low fee to get all of the benefits like support, or updates. In this technology time, the product should be expected to update continuously, use this to get the latest version if we update our product)
Best regards.
Have a nice day,
Vincent

Hey if i buy it... i can use it on 2.3.1?

Hi Erick,
Currently our template works well with magento 2.3.0