It's not a simple subject to approach but I'll try to sum up a very long, boring and complex subject into a nutshell.
Theoretically, customs taxes are put in place in order to protect the national industries by making imported products as expensive as those who are internally manufactured.
In the EU (where I live), these rates are set in a commonly agreed table, for all member countries to apply on each respective territory, to all inbound goods, although some exceptions apply. US, to my knowledge, charges outbound goods, which I am yet to understand why.
Besides the customs tariffs, which are EU taxation and a direct source of income for the community government, national taxes can and most often are applied on top of the customs tariffs, like VAT and other special taxes. In my country, as an example, tobacco and liquor have special, separate, taxes to be paid before the product is cleared for the internal market.
Let's take a look at a few examples:
a) importing electronics components (motherboards and general purpose electronic parts)
customs tariffs: €0 (Europe does not have a leading electronic industry but is mostly an integrator and consumer, so no charges are applied)
VAT: 23% (calculated on the overall value of the package)
TOTAL: (50+5)*23% = €67,65
Motherboards from abroad, unless we are dealing with very specific/high end model, usually do not make it worth all the trouble. Other expansion cards, usually do. I once ordered a USB3.0 expansion cards, with 5 ports, for a friend and it cost me around €15. National market: €55.
b) importing a set of bed linens
cost: €50 (good quality Egyptian cotton linens will cost this and even more)
customs: from 8% to 17% (for this example, I'll add 12%)(there are lots of clothing and related products manufacturers in Europe)
TOTAL: (50+10)*12%*23% = €82.66
On top of this, you may also have to add the fees for handling. National post service charges €12, fixed price. Third party companies make each its own.
Overall total: €94.66
For such a specific item, it would make a great buy. Egyptian linens are well over €200 a set in my country. You would save around 50%.
If you would like any other practical example, feel free to ask. If it's within my knowledge to answer, I shall try to do so.
There are, as I mentioned, a few exceptions. Parcel valued less than €22, in general, and considering the items, will not be stopped at customs and parcel with commercial value less than €150 will not be have customs tariffs applied but only local taxes, like VAT.
Alcohol and tobacco will stop, no matter the value of the parcel. Guns and ammunition can only be imported with special government permits and through authorized gun dealers and besides all normal customs proceedings will also face mandatory police inspection, with additional fees to be charged, and some guns and ammunition are simply banned to enter my country, in respect to national legislation. My country has one of the most restrictive gun laws in Europe, to my knowledge.
Prescription drugs and dietary supplements require a special import permits, who must be separately required at the national equivalent of the USFDA, with separate and additional costs. Some products, are, of course, banned to enter the market.
I could on but enough is enough.
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