Yeah, I see why you are skeptical. Kredit Karte is legit, but poorly executed. They have commission based partnership with 1and1. Its up to you if you like to use them. Really you can use any credit card processor like Authorize.net or even Paypal Merchant if you are willing to invest time and effort to integrate with their API with 1and1 ecommerce.
If I was you I would learn how to use popular carts like ZenCart, Oscommerce (or volusion.com build and enhanced on Oscommerce platform) or Magento. Magento is known to be high on your server resources, so maybe not the best option if you are planning to have 5000+ unique shoppers on your site. All of them have themes and can look however you want them to, plus will already integrate to popular credit cards processors and shipping companies.
All these carts can be installed with your shared hosting account and frequently available as 1-click installs with other hosting provides, so you don't have to play around with databases setup and php installations. Here's one: http://www.webhostinghub.com/web-hosting-features.html
If you cancel with 1&1, you may loose your domain if you have "60 days no transfers lock" after the registration of the domain. If your domain older than 60day, you should have no problems transferring it to a different hosting company.
Let us know if you have more questions.