The final prices are listed
here. It's open to EvilDragon to update those if he needs to, but so far they've remained the same as when I placed my preorder.
The process I expect is ED will put up a page for each type of Pyra, you can put this in your cart and add the vouchers you have (including the pre-order voucher) when you take it through to payment if not before. No need to extra vouchers, unless you want to pay extra early (which some of us have done). But if you've got enough cash handy when it comes to release and don't need to preload, then just select your option, add it to cart, go to payment, add your pre-order voucher and the right amount should be charged.