It looks like you ordered are PA50D system which features a rotary vane compressor so that means your system came with 1/2" tubing. Here is a direct link for that 1/2" airline:
The 3/8" tubing comes with our rocking piston kits. The piston compressors have more pressure but less volume and the additional pressure allows us to utilize the 3/8" tubing for those systems.
The rotary vane compressors give you more air volume but less pressure and using the smaller tubing could cause back pressure and the system would not work very well and there is the potential of damaging the compressor over time due to back pressure.