First of all, IONOS is not a free domain name service provider in general. We usually have very competitive pricing for our domain customers; however, you will not be able to enjoy free domains all the time. The reason we can occasionally give away domains at little or no cost is that our large customer base enables us to attract a lot of registries that work with us.
One thing we learned in reviewing the services listed here (and many more) is that even though the packages are very similar, they are not identical. Some are more security-focused than others, offering anti-spam and anti-malware tools. Others offer a variety of email marketing tools. While most of the hosts we've reviewed have built-in e-commerce, you may want to consider using a more-robust third-party online shopping cart application like Shopify instead.