p e d,
In order to reproduce this issue, we need more information. When reporting a problem such as this, we also require browser type (ie FireFox, IE 6, IE 7, etc.) and version. We have tested purchases using IE 7 and FireFox (version 2.0.0.4).
Your post also does not tell us if you were trying to purchase a product off of WinCustomize, or from www.stardock.com . Please also include what product you are purchasing.
As highlander noted, there you are using multiple email addresses, which is ok. However, this can become problematic if you are using Stardock Central to download the products and have not merged your accounts.
From the sounds of your problem, it is not specifically due to multiple accounts, but we can certainly help you merge your accounts to ensure that downloading of all your purchased products is seamless.
Sincerely,
Stardock Staff