How to bypass forms-based authentication

Hi there,

I'm building my first web site which I want to secure until I'm ready to go
live. I've therefore put up a temporary logon screen using forms-based
authentication (i.e., the "mode" attribute under "authentication" is set to
"Forms" in my "web.config" file and I go through a logon dialog whenever I
access my site - the dialog will be removed when my site is ready). Is there
a simple way to allow a user from a specific domain to access my site
without going through this dialog? That is, I'll be selling various products
from my site and once purchased, my payment provider's server needs to
contact my site so it has to get past the dialog (in order to access a
particular page which I've configured at their end). Note that I'm
relatively new to ASP.NET and web development but a very experienced
developer otherwise. Thanks.