Need suggestions with a website I’ll be building (Use Whmcs or CMS system)

I?ll be creating a website to sell online services, let?s say 3 different services:

1. Facebook services
   1. Facebook fan page likes
      1. Different price packages
      
      
   2. Facebook photo likes
      1. Different price packages
      
      
   3. Facebook ? [status, videos, comments?.] likes
   
   
2. Twitter services
   1. Twitter Followers
      1. Different price packages
      
      
   2. Twitter retweets services
      1. Different price packages
      
      
   
   
3. YouTube services
   1. YouTube views
      1. Different price packages
      
      
   2. YouTube subscribers
      1. Different price packages
      
      
   
   

So best thing for me is to have a shopping cart/order/support management system for all the proper management of the orders that works mostly this way:

• When client selects the specific service he likes, he can choose in between the different packages offered for each one
• He will also need to add the url he will like the ?Likes? for example or the ?target url?
• He will be able to choose how to pay (the different integration payments added to the shopping cart and then pay
• If his order is processed correctly:
  • Order will be displayed as successful and prompted to check his email for further instructions
  • An email will be sent to the customer email and a login (email) / password will be sent to him to access the client?s area for his order tracking / tickets / etc.?.
  • The order should be added to the new orders in the system or to his old profile orders if he is an old customer
  
  
• His customer client area should have the following:
  • Orders area
    • Order number
    • Date ordered
    • Service ordered
    • Target url of his service choosen
    • Approximate time deliver
    • Status
      • Order not yet started
      • Order in process
      • Order delivered
      
      
    
    
  • Client area (to view/update his details)
  • Submit ticket
  • History of support tickets
  • Pre-sales support
  • Livesupport (just link to the actual site to the livesupport)
  
  
• In between every change of status of the client order, let?s say if it?s just been paid and the status is changed to order process started he should get an email telling him we started to deliver his order, same from started to delivered he should get an email telling him his order has been delivered

It must be really SEO FRIENDLY because I?ll do a huge SEO campaign for this site (for example in the facebook likes section to have be able to edit below the offer packages a lot of text or any form of html to add unique content)

What do you think?
Should I use something like:
Whmcs
Or should I use a CMS like:
Magento, Prestashop or OpenCart

Go for WHMCS and you won't regret with anything. It's only $18/month but it can be a huge life saver. It's a complete billing and ticket system and it can be integrated very easily to any design.

My VOTE GOES goes for WHMCS

Whats pros and con could it have vs the cms that are mentioned in the pool?
thanks