Never use a low pressure radiator cap. It is important that the pressure in the radiator be maintained for proper circulation of the cooling fluid within the engine and the system. Low pressure caps should only be used in an emergency to get the vehicle into the shop.
Use a certified auto repair professional, not just any old mechanic, especially if your vehicle is new or under warranty. While a smaller garage or mechanic may be cheaper, using them can invalidate your warranty and may lead to problems if the repair is not done correctly.