Some products are limited in quantity, due to air transport safety regulation regarding built-in battery. The postal service says that there is a potential risk of fire on board, if too many of such products are stacked in the same package. 

Here are some examples for the limit:

1.Electric hand fan, USB-charged hand warmer, light stick power bank: DHL - up to two pieces per parcel (order). Korean domestic shipping - no restriction.

2.Kihno albums, light key ring and voice key ring: small packet / K-packet - up to two pieces.  DHL - No restriction. Korean domestic shipping - no restriction