Hi Mike,
You have come across a limitation of passing the CategoryID in the querystring. Currently passing the CategoryID only filters the Products on the BuyNow module but it hasn't been programmed to select and highlight the Category in the Category module. I do think that it would be useful though. I know a few of our clients have developed their own custom categories modules using Jquery to achieve what you are looking for but right now this isn't supported in the Cart. If you would like to be provided with a fixed quote to develop the ability to select and highlight the Category as well as filter the Products, please submit a Quote Request at the following link -
http://www.smith-consulting.com/servi...
Thanks!