The JP servers' AC sales are often discounts, either on individual purchases or as bundles of two regular AC purchases but at a lower price than the combined price of the bundle. Often they also add bonus items in addition to that lower price.
(Source - bumped.org)
Global AC Shop sales which tend to run through the first 3 days of each month tend to lump together two usually unrelated AC shop purchases at the direct price of both and then just slap on a couple Tokyo Gold keys. Global has never offered Inventory Expansion (10) or Character Storage Expansion (50) at a discounted price.
Back in June through August I agreed with that strategy, saying it would be too soon to devalue those purchase options during a sale, as that would make the sale prices be the new baseline in players' minds and instead of potentially buying them off-sale they'd just wait until the next time they are discounted to that price.
But the next AC sale is likely to be at the start of February, half a year on from that. NGS is coming, everyone is being hyped up for the engine update that was originally planned to be part of NGS itself in February, and NGS itself is said to be arriving by Spring - which PC players have unpleasant memories of knowing can mean late May, true, but that's still only 4 months away now.
NGS has been said not to use PSO2 inventory or storage. There will likely come a time when players just won't care about the size of their PSO2 inventory or storage. The opportunity to have anyone to sell these expansions to is disappearing, so it is probably good time to consider discounting them for the players who might still take up those discounts.
Alternatively if NGS is going to take over all item space consideration (e.g. if PSO2 Mission Pass will be migrated to NGS or replaced by a different form of NGS reward system) it may be worth simply permanently discounting the AC shop purchases that only affect the PSO2 side once it is officially switched over to Long-Term Support mode.