While browsing the in-game store, many God Crystal (abbreviated as GC from here on) packs are offered for sale. The store only sells GCs at nominal price which is very expensive but they often have 'discounts' which appear as free GCs when you check the pack description.

The catch here is that most of the discounts shown in the store are applied over nominal price, while the actual GC price is valued by the most expensive stable GC pack. For an exaggerated example which is simpler to understand: Buying 80 packs of 24 GC (1920 GC total) spends roughly the same money as buying 1 pack of 3000 GC, with vastly different GC payoffs. Therefore, when asking yourself whether or not a pack is worth it for its GCs, it is useful to compare it to the maximum GC pack in store instead of the nominal price.

As a note: Nitori's Delivery is very efficient in terms of money, but its GCs are spread over 30 days, so consider investing in it early if you want cheap GCs.