The women, who were on a shopping trip in Charleston, South Carolina, returned home to Statesboro, Georgia, and gave the Lego to a young child.
They realised something was seriously wrong when the child opened the box and found a package of meth instead of Lego.
The quantity of methamphetamine had an estimated street value of US$40,000 (NZ$60,790). Although the women were unaware of the concealment, they had unwittingly committed a felony by transporting the disguised drug.
The women handed the meth to the police and an investigation with the Drug Enforcement Agency took place.
Authorities confirmed the unsuspecting drug mules were not responsible as the meth was clearly unbeknownst to the trio, the Daily Mail reports.
Authorities believe the contraband was deliberately sent to a wrong address or an empty home by the drug dealer with the intention of someone picking it up.
The Statesboro Herald confirmed that dealers will often ship their contraband to abandoned addresses for someone to collect, despite delivery workers' reluctance to leave packages without the intended recipient.
The US Postal Service is known to auction off packages that go unclaimed. Authorities agree this is likely to be how the meth, disguised in a Lego box, ended up at a post office auction.
The Daily Mail reports that South Carolina had the second-highest number of concealed meth labs in the US in 2016, according to research.