Within the EU, there are specific border crossing charges that will apply. Ask the service provider to explain these to you. It may also be possible to take your rental to non-EU member states, ask your chosen service provider. However, you need to notify the agency when you book, as your car may need specific insurance or extra registration. Make sure you discuss your requirements in detail, fines can be very heavy, with severe consequences, often arriving many months after you arrive home. Search car hire on KAYAK