I inherited a site with a similar issue. They have a main web-site and then it is linked to an e-commerce site to handle the transactions. They don't have time to maintain either so I do. They certainly have the rights to maintain either, they just don't. This is also key. The shopping cart is an open source program. They are more than welcome to find instructions on how to use it. A web design company shouldn't keep your updates hostage.
That being said, the main site gets the traffic and then when someone wants to order there is an easy link to the shopping cart page to complete the transaction.
So I don't see a problem with creating a site
XYZ Consulting that gives the product details, etc. (
SEO optimized), and then just provides the link to the ecommerce site.
Of course, you would want the shopping cart to remember what they have added if they are going back and forth. That could be a problem.
But if they need turbo-orange widgets and you have xyx.com/turbo-organge-widgets and than that links to the widget store and then they continue to shop around at the store for other items, you should be fine.
cd :O)