Saw the post that pointed out you need to apply the code via a single toon, so I tried to use SAVE15 to buy two lots of character bank space upgrades (to go from 100 to 120 then to 140). Applied save15 and the saving could be selected on both upgrades. Selected the first one which reduced it from 595 to 506, when the screen refreshed - I now had 120 storage space - the saving was no longer there and couldn't be reapplied, so the next one cost the full 595.
I had assumed (incorrectly obviously), that if I chose save15, the next 5 items I bought on the same toon without logging out of the store would be selectable for the discount. I wanted and was always always going to buy them both, so I still made a saving, no issues with that. I just need to remind myself to either just use the save30 or choose an upgrade that has a selectable number next to it (such as 5 true wood of bloods) next time this offer comes around.