"We sold him six tonnes of fertiliser, which is a relatively standard order," Oddny Estenstad told AFP, adding the delivery was made on May 4.
Fertiliser can be used as an ingredient to make bombs.
Estenstad would not give details as to which products were supplied or say if they could be used to make explosives.
On his Facebook profile, the suspect identified by Norwegian media as Anders Behring Breivik, a 32-year-old ethnic Norwegian describes himself as the head of Breivik Geofarm, an organic farm.
Estenstad described the fertiliser order as "small" and "usual" for vegetable producers and explained it was routine for producers to buy fertiliser at this time of year.
After the downtown Oslo bombing, in which seven people died, the suspect is believed to have taken a ferry to a nearby island where the ruling Labour party's youth wing was holding a summer camp.
Wearing a police uniform, he gathered youths around him and started shooting, killing at least 84, in the country's deadliest tragedy since World War II.
