As of today's maintenance the Event Items can now be sold to the NPC to free up some space on the inventory.
the previous drop rate is good tho before the last maintenance the only problem is the disposal of excess materials. now after 8 hrs of grinding still no summer box drop.
The Summer Gift Box drop rates have been scaled down similar to the Spring Gift Box and it has been tested to drop on PvM.
You will have to send in a Support Ticket and make sure that you have access to the email on your account.