Isn't it possible to add several bundels to your shopping cart? When I press buy now the only option is process order!

As it seems , not. While you can put as much individual games at -50% as you want in your cart, the bundles such as D&D or Omerta have to be bought one by one. I suppose it has to do with the calculation of the bundle price reduction. ( If the condition of the bundle is not exactly met, the system would only apply the general -50% instead of the more genrous price reduction )

A small inconvenience, but very minor. As a consequence I had to make three separate purchases yesterday ...

I bought 4 bundles in one transaction yesterday. Just go to each game's page, add to cart, and if you add every game you don't already own in the bundle, you'll get the bundle discount.

It's done! :)