The legal situation
If you have concluded a contract at the front door, the contract is indeed effective, but generally you can revoke the contract. By revocation you are not bound to the contract anymore. For the revocation you have a general time limit of 14 days, from the conclusion of the contract on.
If a product has been purchased, the time limit begins, when the goods have been delivered.
If the consumer was not informed about the revocation, the revocation-right expires not later than 12 months and 14 days.
The revocation has to be declared towards the contracting party - The best is to write a letter, which states, that you are revoking the contract.
It is not enough to just send back the goods. The revocation requires no justification.