Michael and Alan Burns' illegal mail-order service saw users across the world being supplied.
The pair's "warehouse" was at a storage complex in a quiet street in Paisley, Renfrewshire.
A judge heard how almost £80,000 had been paid into the brothers' bank accounts.
The brothers admitted at the High Court in Glasgow being involved in the business dubbed "Amazon for drugs". They were remanded in custody and will learn their fate next month.
The court was told how a website registered to Alan Burns, 24, of Dumbarton, offered for sale cannabis, diazepam and a psychedelic drug known as 2C-B. Users simply clicked to make a purchase before drugs were packaged and posted.
Strathclyde Police tracked the brothers as part the Operation Bismark drugs probe.
Prosecutor Kath Harper also told how the pair regularly visited a container park in Abercorn Street, Paisley. On February 9 last year, staff at a sorting office found a parcel that reeked of cannabis, sent from the Springbank Road post office.
Police swooped and arrested the pair at the post office, where Michael Burns, 27, of Neilston, was caught with Jiffy bags of white powder.
Containers at the storage unit were also searched where £34,115 in cash and large stash of drugs were seized.
Judge Lord Burns deferred sentencing for reports.