The recommendation now heads to the NLRB's regional director, who will issue a formal decision about whether to toss out the election results.
The International Brotherhood of Teamsters will reportedly vote Thursday on a decision to organize Amazon workers.
Amazon lobbied top USPS officials to set up the box, which the union cited in its challenge to the election results, Bloomberg reported.
The NLRB said evidence provided by the Retail, Wholesale and Department Store Union warrants a hearing to determine whether Amazon acted illegally.
A group of workers at Amazon's Staten Island, New York, warehouse said Monday they had started a movement for an independent, worker-led union.
The RWDSU filed 23 objections against Amazon on Friday. The objections will now go to the National Labor Relations Board for review.
Jeff Bezos is on the defensive about how Amazon treats its employees following the failed union vote at Amazon's Bessemer, Alabama, warehouse.
Levin co-sponsored the Protecting the Right to Organize Act, a landmark piece of legislation that would make it easier for workers to form unions.
Experts say the Amazon unionizing battle reveals a broken system that makes it harder for workers to organize.
Experts said the Alabama vote may push regulators to look more closely at how giant tech firms like Amazon exert power over workers.
Before the result of the Amazon union vote was announced on Friday, we spoke to two employees about the reasons they voted no.
The NLRB has paused counting until Friday morning, with votes against forming a union leading 1,100 to 463.
The union said the mailbox could make it seem as if Amazon itself would directly see the ballots - a move that could deter employees from voting.
Hundreds of those ballots were challenged, mostly by Amazon, according to the RWSDU. The public voting could begin Thursday.
"We've created a workaround to game the system," says one Amazon driver, who eats lunch while driving and has had to pee in water bottles.
From monitoring drivers with cameras to hiring Pinkerton spies to surveil European warehouse workers, Amazon keeps a close eye on its workers.
Amazon's signature smile logo was created in 1999 and has stuck on boxes mailed to customers ever since.