I just checked their download of the design. I contains all the Gerber files but also the Eagle file. You can download a free version of Eagle which can handle a pcb of 10cm x 10cm so a 5x5cm version is no problem. Eagle can always generate the Gerber files for you again.
I ordered a PCB in September 5 x 5 cm. It cost me including shipping US$ 13.90. And I got 11 PCB's. Counting always seems a challenge with the Chinese
But it makes the cost for one PCB only 1.40 USD. About 1.10 euro......