If you read this guideline, because you have a problem with an online shop, you should familiarise yourself with a very important consumer right and its statutory limitation, namely the power of revocation:

The power of revocation gives you the right to return the purchased product to the online shop without giving reasons, but is limited to 14 days after delivery of the product. Furthermore, the online shop may grant you the right to cancel the contract (§ 312 D BGB;  § 355 BGB;  § 1 BGB InfoV;  § 3 BGB InfoV).  However, the statutory limitation begins to run after the following procedures are satisfied: you were duly instructed about your consumer right, the online shop fulfilled its duties to supply additional information pursuant to § 312 C BGB and/or § to 312 e BGB, and you received the product (§ 312 D exp. 2 BGB).  In some cases, the calculation of the statutory limitation can be difficult.  If you have questions, contact your consumer advisory board or an attorney.  However, you should revoke the contract in writing.  It is sufficient that the posting date (sending) falls within the 14 day period.  Furthermore, you should make use of certified mail and keep the mailing receipt.