FWIW, for export/imports you have specific codes, for example, detailed in the Harmonized System Codes (HS Code 2017) - that's what is negotiated in trade agreements. For example, I guess this could be exported under (rough guess, several options imho)
851840 Amplifiers; audio-frequency electric
Now, each pair of countries/regions (say US - EU) can have different rates on the same items depending on the direction. So, typically a package sent from US to EU will be subject to a customs duty dependent on its code number. There is some ambiguity in the code and some things can fall under several different codes. Optimizing codes as to lower fees is an actual job.
The big no-no are exporting stuff with a wrong code, no code at all (although the shipper will usually catch missing codes) or under value - this is typically what happens when you hear stories of people buying a $10 trinket and ending up paying $200 in fees/fines/taxes. If there is something wrong, your customs/post office may end up charging you a "stopping" or a "processing" fee.
Assuming the code is correct, one would end up paying base (price + shipping price + duty fees + insurance fees)*(1+applicable VAT rate)
Assuming incorrect code, it would be (price + shipping price + duty fees + insurance fees + processing/stopping fee)*(1+applicable VAT rate) + eventual fine.
Assuming no code, the same as above or bounce or total loss depending on the mood of the guys who process things and legality/duty fraud attempt. Customs Clearance Fees are usually not applied when everything is perfect (I guess you could sue/complain if they were) but whenever a wrongly labeled package is sent, customs can intervene and charge you for their "specialist" time.
When the deal is direct to customer, there is usually some tolerance, but relying on it is a bit like playing the lottery, especially on high value items.
And, of course, as far as shipping is concerned, volumetric weight matters more than actual weight.
A reference for EU
https://ec.europa.eu/taxation_custo...s-online-coming-from-a-noneu-union-country_en
As an aside, this is mostly why a no-deal brexit would be so disruptive. EU to EU has become child's play compared to what it was before. And, of course, a very complex system favors large entities, think amazon, and put a severe load on the small guys...