Magento: Enter Voucher Code in OnePageCheckout (OPC)

Hello, in a standard Magento installation Customers do have the option to add a Voucher Code they received in the "Cart"-view page only. I would like to give the the possibility to enter their code in the One-Page-Checkout Page (OPC), too (e.g. in a box in the right columns above the one that shows their checkout-progress. As many costumers go to checkout directly without visiting the Cart-Page this would be great. The possibility to add the Code in the Cart-Page should stay intact nevertheless. Optional: The task written above is crucial for this job. So please only bid if you can do this. But there is one optional task with this job: It would be great if customers would not only have the possibility to enter the Code in Cart and OPC but also in the Mini-Cart that I display in my shops sidebar. If you could do this, too, please give the the addtional costs for this feature in a Private Message. Thank you! IMPORTANT: You will have to work in a shop that is already running live! There is NO Development-Enviroment. So please only bid if you could do the work in times that would be night-time in my region (Germany, CET - Central European Time). Thank you!