Hi, I'm looking for an easy-to-use and very user-friendly open source (if possible) shopping cart/ecommerce system that I could use to sell several services to clients visiting the web site. Moreover, I need the following functionality: The client should be provided with a list of possible upgrade products useful for the items s/he has in his or her cart. In other words, I would prefer to have an automated follow-up sales module. Thank you in advance for your advice.
If your looking for open source shopping carts then CubeCart, OsCommerence and Zen Cart are your choice. They provide a simple solution for people, like you, which want to have their very own shopping website. All 3 cart systems have free upgrades or add-ons which could help you customize your site. With our web hosting we offer these shopping carts and I personally will install one of them for you for free. Once installed you have to build, customize and promote the website on your own. I'm sure you could find a add-on for that kind of functionality. You should do some research and find out.
I´ve tested all three of the earlier mentioned shoppingcarts and decided on ZenCart for it´s user friendliness. It´s community (addons and forum) is not as large as the one of OSCommerce but sufficient. I use it for sellling ebooks, scripts, software etc. and ZenCart does all that I need (I did add a few free extra features).
Avactis shopping cart has the "Related products" and "Featured products" features that do just what you need. This is a good script, was easy for me to install, setup and working fine now.