Of course this are pros and cons to the above as well.
For instance if you buy directly from shipments you can get it immediately, where as farming would take apparoximately 5.5 days and also depends on RNG (granted everything though even out to the demoted drop rate in the long run).
Farming for them will save you almost half the crystals (if you are using 50 crystal refreshes only). If you were to use 50, 50, 100, you're total would be apparoximaely 1,250 crystals for a drop rate of 1/3 which would only save you less crystals, but at the same time saving you 2-3 days of farming.