Hi, It will not let you purchase an Item with the same paypal account that your paying to. What I did was use my wifes paypal to buy a dummy product and then sent the amount back to her paypal.
I also wanted to mention to you that if you just want to charge a flat rate for shipping and dont want to use weight as a measure for prices. You wont need to get the usps ID. You can specify a flat rate shipping and make the price whatever you like in your shipping module.
If you have anymore problems, or questions, you would be better off to go on the vstore forums. It is very helpful and the people there would be more qualified to help you than I would. It has been over a year since I even messed with the modules in the store and my recollection of how everything works may not be as accurate as I remember it.
Fortunately, the things you have asked me so far I remember somewhat...Thats a miracle in itself...lolol. Its amazing what the brain retains...I had no idea I would even remember some of this stuff..lol
There have been alot of added features since when I first started, like the google crawling tool and other stuff. You will find all kinds of info about stuff in the vstore forums.
Hope this helps you!