Magento Extension for round all computed prices

Hello For an online shop in switzerland, i need to round all computed prices (such as coupon prices, discounts or any shipping/payment fee) to the next higher or deeper 5 step rapping (cent), cause in switzerland the currency have only 5 rapping (cent) steps. This should be solved in an extension which just override the function are needed from magento. Hardcode in core is not allowed. I had tendered this project once, with an bidder who didn't done the job and wasted my time, i have not much time left in my project timeline, so please only bid if you realy are interessted and have enough time! and please give a realistic deadline.