I have a merchant account at Verisign and I am currently using a free
shopping cart service called Mals for my website at
http://cableorganizer.com.

Unfortunately, the site I am using is somewhat limited as to the
discounts I can offer and the shipping zones and calculations.  I am
looking for a more robust shopping cart system (software or asp) that
I can run that would be fully integrated with Verisign.

My budget is $10 - $15 per month maximum.  

I would like the purchase information and a link to find more info on
2 or 3  services and 1 or 2 software packages, that meet my criteria.

There are several choices in this market and arena. Most of the robust
shopping carts out there are probably going to cost anywhere from
several hundred dollars to two thousand dollars. Because I enjoy
finding great products that are aware that are free or have a minimal
cost.

1. I would definitely recommend looking into OSCommerce. You can find
this very nice and robust shopping cart that is GNU at
www.oscommerce.com. And they have implemented the Verisign Payflow
module for this shopping cart. Please look at the showcases they
present and other live sites with this shopping cart. It is by far the
best shopping cart I have seen without costing you money for the
software. I will note that it does require a server running PHP and
MySQL. A free product that has just about any feature you need for a
shopping cart.

2. Another great shopping cart that is on the market is Americart. I
have not dealt with them personally, but they offer a shopping cart
service that they host on their servers. The cost is $249 a year or
$20.75 a month. They do have a built in discount system. They also
offer automated realtime shipping rates for FedEx, UPS and USPS. Their
website is http://www.cartserver.com/americart/. They have an online
faq as well.
Here is another link to answer your questions concerning Payflow
http://www.cartserver.com/americart/faq-vp.html

3. You might also checkout RedHat Interchange 4.8. It is also a very
nice shopping cart for free that is very robust and provides the
options you are looking. It also supports the VeriSign Payflow you
were looking for. The website is interchange.redhat.com. To see this
option, look under the Administration Tool, Wizard and payment
options.

There are many different flavors out there. You will need to decide a
couple of things. Whether or not to host your own site. Which server
side technology if any you are going to use (.ASP, .PHP, or .CGI) and
what database you are going to use. I am sure there are tons of other
resources that I have not mentioned. If you would like to see what
other scripts are available I would also check hotscripts.com.

I hope this helps.

I guess I should have mentioned that I do not control my web server. 
It is hosted on Sun Systems machine and runs Apache.  That's why
shopping cart service is better than software.  I can add Perl and CGI
scripts to my server but I can't install any software on it.