Melting water absorbs 334 kJ per kilogram and does so at 0 °C, which makes a frozen bottle a cheap and effective cold source. But you cannot choose its temperature. Where the load has to stay at -18 °C, the bottle does not protect the product, it only thaws it slowly.
The bill for the free option shows up on the product. Water expands as it freezes; a bottle filled to the brim deforms, cracks and empties onto the product as it thaws. Its outer surface sweats inside the container, the packaging gets wet and the cardboard loses strength. Leave an expansion gap when filling and check the crack and the cap after freezing.
Frozen food, medicines and specimens, and any setup with direct food contact, do not use frozen bottles: these jobs require the working temperature of the cold source to be documented, and a filled PET bottle offers no such document.