Create advanced PHP stock management system

We have 2 OpenCart stores. We require an all in one system that will manage stock and orders from both OpenCart stores externally. The system must have an Add Stock feature, which will allow us to add stock to OpenCart 1 and a Transfer Stock feature which will allow us to match a product on OpenCart 1 with OpenCart 2 and transfer stock. We will need to be able to add several custom fields when adding products to our OpenCart website. One of these fields will be for "best before date" which the system must be able to track. We must be able to collate and print invoices from the system. We require an order picking section, where our staff can verify orders are correctly picked and packaged by scanning each item with a barcode scanner. The system must be able to add and manage stock, transfer stock between OpenCart systems (one site is for wholesale, one site is for retail), view and print orders with barcode, retrieve orders by barcode, verify orders are picked correctly by barcode, and have the ability to complete stock [login to view URL] system must also compile charts based on sales statistics, traffic and conversion. This system is relatively complex and will require a competent and experienced PHP expert with knowledge of ecommerce, in particular OpenCart, with a good knowledge of English and capable of carrying out extra tasks. A strong knowledge of mathematics is essential. This is just a simple explaination of the job. The job will have more detail.