Prestashop Bug Fix and a Module modification

Hello, I need 2 things: 1) to fix a bug in One Page Order of Prestashop. I have an external shipping module, which doesn´t update it´s carrier cost when I change/add/update delivery address. Additionaly sometimes, when I change adress, I receive a technical error. I do not know what all to fix, but posibly the theme template/JS/AJAX and my custom module. 2) to modify the Prestashop Followup module functionality in order to be able to set each rule only for selected user groups. Expected delivery is next 48 hours. Thank you.