According to police, the box was mailed from the Netherlands, intercepted by Homeland Security and tracked to a home on the 3000 block of Iowa Avenue.
Investigators served a search warrant at the home on Wednesday and found more ecstasy pills along with Xanax pills and similar postal packaging.
In all, authorities found a total of 1,140 ecstasy pills and 90 Xanax pills with an estimated street value of $23,250.
Sung Hee Jeong, 21, of Tujunga, was arrested. She was booked on suspicion of possession of a controlled substance for sale.
The investigation is ongoing. If you have any information about this case, you're urged to contact Riverside police.