Add stripe as a checkout option with PMPro.

pmpro_stripe_checkout.php

function my_pmpro_valid_gateways( $gateways ) {
    $gateways[] = "stripe";
    return $gateways;
}
add_filter( "pmpro_valid_gateways", "my_pmpro_valid_gateways" );

function my_pmpro_checkout_boxes() {
?>
    <a href="?level=<?php echo intval($_REQUEST['level']);?>&gateway=stripe">Checkout with Stripe</a>
<?php
}
add_action( "pmpro_checkout_boxes", "my_pmpro_checkout_boxes" );