Based on the applicable law, which is typically the customer’s domestic law (for B2C within EU, see also Legal Basics 🌎 Which law is applicable in CS cases? ), domestic laws within the EU require sellers to cover the expenses for shipping items back for additional services, such as warranty services.
General Terms and Conditions that exclude covering shipping fees in the event of warranty service within the EU are not allowed!
→ Exception: Warranty exclusions (see also: 🚍 Transport & Processing Costs for Warranty Exclusions )
For shipping outside the EU, it may be reasonable for the supplier to refuse warranty services due to high costs for overseas shipping. Offering a refund minus the loss in value is reasonable.
If the supplier chooses to reverse the contract, they shall however cover the transport costs for returns.
In conclusion :
The supplier is responsible for covering shipping costs for warranty services and returns, unless other specified for non-European countries previously excluded in the GTC.