A Munich shoe store manager received an unexpected return item in the mail that on first suspicion he thought may have been a bomb, but turned out to be sperm, German daily Bild reported on Friday.

"It really looked dangerous. I thought for a bit that it was a bomb or liquid explosive," the store manager said.

The manager opened the one-meter (3-foot) tall package to find a nitrogen tank and a label reading: "Warning, hazardous material. -175 degrees Celsius. Liquid Nitrogen."

Inside was information about the donor: a 178 centimeters (5 feet 8 inches) tall, 82 kilograms (181 pounds) , blue-eyed, blonde haired man with A+ blood type.

The package also reportedly included instructions on how to use the sperm for at home insemination.

The delivery originated from the Danish company Cryos International, the world's largest sperm bank.

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Mix up

Oddly, the return address sticker was for a shoe customer in the southern German city of Constance who had no idea how there could have been a mix up.

"Our family planning days are long over. We sent shoes back, but a sperm donation would be news to us," the customer told Bild, adding she didn't know how her return address ended up on the package.

German shipping giant DHL and Cryos International are now investigating the mix up.

In the meantime, the sperm was sent back to the Danish company.

Inseminating at home

Sperm from sperm banks is only used in Germany at special clinics and practices.

Those on the market for sperm can even choose the traits of the donor, from height and skin color to educational background. 

The cost of sperm starts at €2,000 ($ 2,385), in addition to medical fees.  It may take several times for a woman to become pregnant with donated sperm, meaning costs can skyrocket.

Due to high costs and the need to visit a clinic, some women order sperm from the Danish sperm bank and are able to inseminate at home.