Three years ago, Amazon launched a program called Vendor Express to make it easier for wholesalers to get inventory onto the e-commerce site and avoid having to go through the company's invite-only Vendor Central portal.

It was a short-lived experiment.

Amazon said Monday that it's closing Vendor Express. The company told sellers in an email, which was obtained by CNBC, that it will stop taking orders as of May 21, and that the program will become "permanently unavailable" starting Jan. 1, 2019.

"We're constantly looking for ways to improve the selling experience on Amazon," the email said. "After careful evaluation, we've decided to retire Vendor Express and refocus the business on other selling programs."

Amazon confirmed to CNBC that it's ending Vendor Express and said the program's vendors will be "transitioned to alternative ways to sell on Amazon."

Vendor Express was launched in 2015 as a new channel for small businesses to sell products in bulk to Amazon, whether it was their own private label brand or other items that they wanted to unload. Unlike Vendor Central, which is targeted at large brands and sellers and requires a thorough vetting process for approval, Vendor Express offered a quick onramp for smaller merchants to have products listed under the tag, "sold by Amazon."