I withdrew from the contract without giving any reason, but the e-shop did not reimburse the whole amount paid. Can they do that?

You should only be liable for any diminished value of the goods resulting from the handling of the goods other than what is necessary to establish the nature, characteristics and functioning of the goods. Your should only handle and inspect them in the same manner as he would be allowed to do in a shop. In principle, you should be able to open the packaging to access the goods if similar goods are normally displayed in shops in unpacked condition.When returning the goods, you should take reasonable care, for example, by choosing an established transport or postal service provider, to ensure that the goods reach the trader and are not damaged in transit.