Mu-plugin to allow you to apply FREE shipping for specified shipping method in Marketpress, If cart total greater than set minimum,free shipping is applied.

mp-custom-free-shipping.php

<?php
$free_ship_minimum = 100; // minimum total for free shipping to be applied

$shipping_method = 'weight_rate'; // the shipping method for this to be applied on.

/*
 * Changes the Shipping cost on the Checkout Estimated costs page
 */
add_filter( 'mp_cart/cart_meta/shipping_total', 'mp_custom_shipping_price', 88, 2);
/*
 * Changes the estimated shipping total text to Free if cart total is above $free_ship_minimum
 */
function mp_custom_shipping_price( $shipping_line, $this ) {
    global $mp_cart, $free_ship_minimum, $shipping_method;

    $cart_total = $mp_cart->total(false) - $mp_cart->shipping_total(false);

    if ($shipping_method == mp_get_setting( 'shipping->method' ) && $cart_total >= $free_ship_minimum) {
        $shipping_line = '
                <div class="mp_cart_resume_item mp_cart_resume_item-shipping-total">
                        <span class="mp_cart_resume_item_label">' . __( 'Shipping', 'mp' ) . '</span>
                                <span class="mp_cart_resume_item_amount">Free</span>
                </div><!-- end mp_cart_resume_item-shipping-total -->';
    }

    return $shipping_line;
}

add_filter( 'mp_cart/total', 'mp_custom_shipping_price_total', 99, 3);
/*
 * Deducts shipping total from cart Estimaed total if cart total above $free_ship_minimum
 */
function mp_custom_shipping_price_total( $total, $item_total, $this ) {
    global $mp_cart, $free_ship_minimum, $shipping_method;

    $cart_total = $total - $mp_cart->shipping_total(false);
    if ($shipping_method == mp_get_setting( 'shipping->method' ) && $cart_total >= $free_ship_minimum) {
        $custom_total = $total - $mp_cart->shipping_total(false);
    }

    return $custom_total;
}

/*
 * Sets the shipping cost to 0 if cart total greater than $free_ship_minimum
 */
add_filter( 'mp_calculate_shipping_weight_rate', 'mp_custom_shipping_total', 100, 10 );
function mp_custom_shipping_total($custom_total, $total, $cart, $address1, $address2, $city, $state, $zip, $country, $selected_option) {
    global $mp_cart, $free_ship_minimum, $shipping_method;

    $cart_total = $total - $mp_cart->shipping_total(false);
    if ($shipping_method == mp_get_setting( 'shipping->method' ) && $cart_total >= $free_ship_minimum) {
        return 0;
    }
}