https://wccftech.com/you-can-now-buy-the-afterburner-card-for-mac-pro-as-a-standalone-purchase/

Apple is now allowing users to purchase the Afterburner card for the Mac Pro separately. If you already have a Mac Pro, you can go ahead with the upgrade.

When the Mac Pro went on sale last year, you had to configure the computer with an Afterburner card at the time of placing your order. The option to upgrade later was not available. But now, Apple is softening things up and is now allowing standalone purchase of the Afterburner card. So if you already have a Mac Pro and want to upgrade, you can do so on March 30th.