You can order/buy more than one AP or PAP using your DC discount.
If you order online, you will not receive the DC discount. The online system is not set up to give any discounts. You can order them over the phone or at one of the Disney Stores, though, and still get the discount. Of course, you can get the DC discount at the parks, too. If you order online or over the phone, you will pay $3 for shipping. If you order them at one of the Disney Stores you do not pay shipping. Most Disney Stores do not have the AP/PAP vouchers in stock and will have to order them for you. This usually takes 2 -3 weeks and you will have to go back to pick them up when they arrive.
No matter where you buy WDW ticket media (including AP/PAP), you will pay the 6% FLorida state tax. You should NOT have to pay any other state's tax on WDW tickets.
Since it is very unlikely that prices will increase before your trip, my recommendation is to just wait until you arrive and buy the passes then. If you want them in hand before you go, I'd order them over the phone. (I've done that many times and not had any problems). You will still have to exchange the vouchers for the actual passes at one of the parks or at DD. It takes about the same amount of time to exchange the voucher as it does to buy the actual pass.
HTH.