The body of Rika Okada was found in a storage lock-up in Tokyo. Investigators also found the 2-metre (6ft 6in) box in which it had been transported from the southern city of Osaka, reports said.
The delivery service that ferried the package -- marked with the Japanese word for 'doll' -- 400 kilometres (230 miles) to the capital had been paid in Okada's own name. The bill for the lock-up's short term rental had been settled using her credit card.
The body of the 29-year-old, who had been missing since late March, had more than a dozen stab wounds, local media reported, but no defensive injuries on her hands or arms.
Police in Osaka refused to confirm details, but reports said a woman who had been at elementary school with Okada had flown out of Tokyo earlier this month using the dead woman's passport.
The schoolmate, who was not named, is believed to have lived just a few hundred yards from the lock-up with a Chinese woman of about the same age.
Both women flew from Tokyo's Haneda airport on the same flight, bound for Shanghai.
Just before she went missing, Okada wrote on her Facebook page that she was going to meet up with an old friend whom she had not seen for a decade.