Even though it is the seller who is to pay the shipping costs in connection with the complaint you cannot just send the goods for repair COD or by similar means. You must disburse the shipping costs yourself and subsequently claim a refund from the seller. Therefore, it is important that you retain the documentary evidence of your expenses.
If the seller denies the existence of a defect and therefore refuses to pay the shipping costs you must first prove that there is a defect in the goods.
You should be aware that other rules apply if you return goods in connection with your exercise of your cancellation rights on distant selling, e.g. purchases on the Internet. Here it is normally you who must pay the cost of return.