If you haven't claimed the purchased items in your mailbox and have not received the first monthly bonus rewards, you can request a refund within 7 days from the time of purchase.
However, please note that refunds cannot be processed if more than 7 days have passed since the purchase, if you have already claimed the items, or for immediate benefit-activating items (such as Town Pass). Please be aware of this limitation.
You can also find information regarding this in the purchase confirmation screen at the bottom of the in-game item purchase page.