While 1&1 does not charge domain transfer fees, a domain registration fee may be due in the following two scenarios.
Transferring a domain between two different accounts with different Account Numbers:
Since the domain is moving between two different accounts, the account gaining the domain will be charged the domain registration fee as if it were a new domain registration.
Transferring a domain between two different packages with the same Account Numbers:
A charge will be applied when the domain is a (free) included domain
in the current package and the package gaining the domain already registered its maximum amount of included (free) domains. Since the domain is included in your current package, but not the receiving package, a domain registration fee will be charged at the time of transfer.
For additional information, you may want to reference: