You can make as many orders from as many vendors as you wish at once, but why on earth wouldn't you be using PGP?! You can encrypt your address for multiple vendors and there are guides on how to do that available in this very forum. A search will turn them up. Whilst PGP isn't required to make a purchase, it is necessary for your own safety. You should be acting at ALL times as though Silk Road is compromised - if you do not, you will inevitably let your guard down. There are countless reasons why you don't want to be sending your name and address to vendors in cleartext. Learn how to use PGP, it literally takes less than an hour to wrap your head around as much as you need to for using it to encrypt / decrypt messages and its value is priceless. Libertas