Sliced Invoices: add additional options to Payment Reminder Days settings

functions.php

add_filter( 'sliced_email_option_fields', 'sliced_email_option_fields_add_reminder_days' );
function sliced_email_option_fields_add_reminder_days( $array ) {
	if ( ! isset( $array['fields'] ) ) { return $array; }
	foreach ( $array['fields'] as $i => $field ) {
		if ( isset( $field['id'] ) && $field['id'] === 'payment_reminder_days' ) {
			// add to beginning of options list:
			$array['fields'][$i]['options'] = array_replace( array(
				'-60' => '60 days before Due Date',
				'-30' => '30 days before Due Date', //etc...
			), $array['fields'][$i]['options'] );
			// add to end of options list:
			$array['fields'][$i]['options'] = array_replace( $array['fields'][$i]['options'], array(
				'+60' => '60 days after Due Date', //etc...
			) );
			break;
		}
	}
	return $array;
}

Note: updated array_merge() with array_replace(), but you'll need PHP >= 5.3 to use this