Japanese antitrust officials raided the offices of e-commerce company Rakuten Inc on Monday after complaints from online merchants about the company’s free shipping policies, prompting its shares to briefly fall by as much as 3%.

A Rakuten spokeswoman confirmed the raid and said the company was cooperating with the investigation.

Companies selling their products on Rakuten have complained that the e-commerce giant was abusing its dominant position and putting pressure on them to shoulder shipping costs.

Rakuten plans to make shipping free for orders above 3,980 yen ($36.26) starting March 18, in an effort to compete with rivals such as Amazon.com.

The company’s shares were down 1.4% at 862 yen.