For someone that claims to be a web designer, you`ve sure been living in a cave for the last; well, for as long as you`ve been in business. But, it is very common for some web design companies to focus on one area of web development and not the other, but by far every web designer / developer that I know have had their share of having to provide an ecommerce solution to one of their clients at one time or another.
But (Im going to echo Roland "VW") and it`s true what he is saying, there are various ecommerce solutions out there that you can use but it all depends on the features and needs of your customers. For example, some shopping carts can handle digital downloads and others can`t or maybe they could but you have to buy the modules individually.
Then you have, the Generic Shopping carts that you can modify to meet your customers needs and expand their functionality as their business grows.. For example: Multi-Payment processing, Image uploads, Client side Admin panel, and Store owner Admin Panel (to upload their own products).
My rule of thumb is, The more features the better, even if the customer doesn`t use all of them, because you never know when the customer wants to expand and the least I want to do is re-visit a web site that I`ve developed, modified, or designed 8 month down the road and try to pick up where I left off.
There are various solutions out there that you can use, but it all depends if you want to use Open Source, or if you want to pay a monthly subscription for the Shopping Cart License.
Finally, If you`re going to develop a shopping cart system... Don`t start SMALL and Grow BIG... It is COSTLY and your business can suffer if you are already ranking and indexing on the search engines. Not to mention if you have customers signing up.
Ecommerce shouldn`t be costly if it`s done correctly the first time.
Web Developer / Owner / Consultant
When starting your own business the need to "know-how" is greater than money!