Adding custom functionality to eCommerce site (PHP, MySQL, Opencart/Mijoshop, Joomla)

Existing Joomla 3 web site with eCommerce installation, MijoShop (Joomla component based on Open Cart 1.5) and MySQL DB, needs to get custom functionality added with medium complexity. Current Mijoshop setup is almost out of the box with some minor functional changes already implemented. PHP skills with OO programming skills required, OpenCart (or Mijoshop) experience extremely helpful, Joomla experience not critical in this project. Basic HTML and CSS. We have a tight time frame, and the project need to be completed (at least 90%) by mid January. Regional freelancers preferred (WI, IL), but all freelancers in the US are considered. Unfortunately we cannot hire international freelancers located in countries other than the United States for this project. The scope outlined below is currently based on some assumptions, and some changes and additions may take place during the project. Therefore, please include how changes would be billed/charged. Desired new functionality: a) Customers can add an unlimited number of Cost Centers to their account and maintain similar to shipping addresses. Cost Center number, descriptive name field, and potentially a "default" check box. b) For ordering, customer should be able to select one of the cost centers during check out (one cost center per order). The cost center info needs to be stored with the order data and visible in the order history, as well as any order confirmation emails. c) Budgetary approval: All orders need to be approved by a pre-defined supervisor: Once an order is submitted, the system needs to look up an email address in a DB table to send a special approval email to that person. Following is an outline of the workflow: 1. Customer (Buyer) submits order (after selecting cost center) 2. Order is stored in the system, but on hold. It cannot yet be submitted to us, the vendor. 3. Buyer gets a confirmation email (with the info that the order is pending approval). This can be the modified original order confirmation. 4. Supervisor gets an email with approval request. Should include buyer info and ordered line items, as well as a link for approval/denial. The supervisor information is derived from the lookup of the selected cost-center in a separate mySQL table and returning the supervisor name and email. 5. Supervisor clicks the link to a new web page, whereas the page shows the order items again, with option to deny or approve. Potentially the supervisor needs to login first to get to the approval page to prevent abuse. It can be assumed that the supervisor would have an account in the system already set up. 6. If denied, the buyer gets an automatic notification email and the order is set to canceled. If approved, Buyer gets an automatic notification, and the system then sends the standard order email to us (shop owner). Presumably the official order date needs to be changed to the approval date, or ideally the approval date can be recorded separately. 7. If a supervisor does not accept/deny after a system-wide pre-set time out period (customizable at least in the source code), supervisor will get a reminder email of a pending approval, repeated in regular intervals (also preset, but customizable in the source code). Possibly the buyer has the additional option to resend an authorization request to the supervisor in the order history, if not too difficult. Upon successful outcome, future customization may be required/desired, new or built on the above.