Allow specification of a discount coupon through the URL, this means we can do away with the coupon field in the checkout process.

discount_url.php

<?php

/**
 * Allow people to specify a coupon code in the URL, and add it to their cart.
 */
function yst_edd_discount_link() {
    if ( isset( $_GET['coupon'] ) ) {
		// Check whether it's a valid coupon, if so, add it to the cart.
		if ( edd_is_discount_valid( $_GET['coupon'] ) )
			edd_set_cart_discount( $_GET['coupon'] );

		// If we can redirect out the $_GET parameter, let's!
		if ( is_singular() ) {
			global $post;
			wp_redirect( get_permalink( $post->ID ), 302 );
		}

	}
}

add_action( 'template_redirect', 'yst_edd_discount_link' );