Items bought from the Item Shop goes to the 'Item Shop Storeroom' (can also be accessed from your inventory by clicking the button. It requires the same password you use on your normal storeroom
Your default storage password is 000000, but you have to change it after the first time opening your storeroom (password requirements apply)
If you have forgotten your password, you can reset it to 000000 by clicking on the gearwheel in your launcher from the account list. It will send you to the homepage where you can press "Storekeeper". An e-mail will be send to confirm the reset and you should be able to access your storeroom ingame after you have opened the link in the e-mail
Changing your storeroom password will give you a bonus that applies to all characters of that account. A Status Icon will appear on the upper left corner. The buff lasts 90 days. When you change the password again, a new random bonus is given. Possible bonus:
Max. HP +1%
Defence +20
Strength against monsters +3%
EXP +5%
Your storeroom is account-bounded, which means it can be accessed and shared amongst all characters on that account
You can unlock 90 more inventory fields if you use Storage Chest
The Item Shop storeroom has a default of 225 inventory fields (5 pages) and can't be extended