refactoring plus fix with line item amount

git-svn-id: https://plugins.svn.wordpress.org/conekta-woocommerce/trunk@1022696 b8457f37-d9ea-0310-8a92-e5e31aec5664
adding new helper

git-svn-id: https://plugins.svn.wordpress.org/conekta-woocommerce/trunk@1022697 b8457f37-d9ea-0310-8a92-e5e31aec5664