Islanders must now pay goods and services tax (GST) on any online purchases imported into the island worth £135 or more.
Previously, the 5% charge was only applied to goods bought for personal use costing £240 or above.
The change to the "de minimis" value was outlined in the government plan for 2020-2023, and agreed by the States Assembly in December 2019.
It was originally due to come into effect from 1 July, but was delayed due to the coronavirus pandemic.
A lower threshold for chargeable goods aims to encourage people to buy locally and allow businesses to compete more fairly with off-island retailers, the government said.