Programmatically alter the WooCommerce cart discount total (and apply a new discount) before it's calculated through the woocommerce_calculate_totals hook. Add this code to your Wordpress functions.php file.

woocommerce-alter-cart-total.php

<?php
/**
 * Programmatically alter the WooCommerce cart discount total (and apply a new discount) before it's calculated 
 * through the woocommerce_calculate_totals hook.
 * 
 * For example, if you want to apply an arbitrary discount after a certain number of products in the cart, 
 * you can loop through that number and increment it as needed.
 * 
 * In this example, I increment the discount by $8.85 after every 15 products added to the cart.
 * 
 * @return void 
 */
function mysite_box_discount( ) {
  
    global $woocommerce;
 
    /* Grab current total number of products */
    $number_products = $woocommerce->cart->cart_contents_count;
   
    $total_discount = 0;
    $my_increment = 15; // Apply another discount every 15 products
    $discount = 8.85;
    
    if ($number_products >= $my_increment) {
        
      /* Loop through the total number of products */
      foreach ( range(0, $number_cards, $my_increment) as $val ) {
        if ($val != 0) {
      		$total_discount += $discount;
      	}
      }
   
      // Alter the cart discount total
      $woocommerce->cart->discount_total = $total_discount;
    }   
}
add_action('woocommerce_calculate_totals', 'mysite_box_discount');

Thanks for a snippet, worked like a charm

Any way to do this with the new Woocommerce 2.3 updates to the discounts class?

It seems this is not working on checkout page. When I am using this code it is working on cart page then click on "proceed to checkout" button and checkout page appears but 100 Rs. discount is not applied on that page. I am stuck. It would be great if anyone can help. Thanks!

add_action('woocommerce_calculate_totals', 'discount_cal');
function discount_cal(){
global $woocommerce;
$dis_val = 100;
if(is_user_logged_in()){
if($dis_val != 0){
$discount = $dis_val ;
$total_discount = 0;
$total_discount += $discount;
// Alter the cart discount total
$woocommerce->cart->discount_total = $total_discount;
}
}
}

I have also tried this (I was assumming cart session also needs to be update)
//try - session discount apply sol- for checkout
$woocommerce->cart->cart_session_data['discount_total'] = $total_discount;

Can't get this to work either. Ended up creating a negative fee in place of discount.

function sale_custom_price($cart_object) {
// Calculate discount amount and return $discount

if ($discount) {
$discount *= -1;
$cart_object->add_fee('Discount', $discount, true, '');
}
add_action( 'woocommerce_cart_calculate_fees', 'sale_custom_price');

Taken from here: http://stackoverflow.com/a/23371512/3233036

However this adds an extra line to the checkout page for Discount instead of using the already present Discount field. Any ideas?

Hey, I'm looking to apply a discount without code. An automatic discount over 100$ ...
Do you know how can I do that ?
Thanks