Extra charges will apply for anyone shopping from UK-based websites from January 1st next year.
The Government's Brexit Readiness Action Plan warns import charges, duty and VAT costs will apply similar to when ordering from the US or China.
It means people will have to pay more for any goods ordered from the UK when they arrive.
A Government official suggested someone ordering a Premier League jersey from the UK would have to pay import when it arrived here.
The Government plan is warning all businesses trading to, from or through the UK to prepare and has announced some financial supports for dealing with extra customs costs.
The document states: "Consumers should be aware that from 1 January 2021 purchases from or through websites in the UK will incur additional charges, duty and VAT, just as for any other third country.
"Administrative costs of export and re-import declarations in relation to returns for retailers will arise."
While it says any plant or animal products will have to undergo checks "and fees may be payable".
"In certain circumstances, EU consumer protection legislation may no longer apply to or in respect of the UK".
In its advice, the European Consumer Centre Ireland notes: "If the UK leaves the EU, you will no longer automatically enjoy the same rights that you have under EU law.
"Whilst UK domestic legislation is currently aligned with EU consumer protection law, this could change in future.
"If you purchase a product online from a UK trader after Brexit, pay special attention to the terms and conditions (e.g. returns and cancellation policy) as these may be particularly relevant in case of difficulty with the law applicable to the contract."
Additional reporting: Sean Defoe
