You might not only have to pay the price on the label.
Taxes and fees are boring to read and talk about. However, as they are a function of where you live, compared to where you buy your kilt, or rather, from where it is shipped to you, they are not without importance and should be taken into consideration.
Being an EU citizen, my knowledge is limited to trade conditions between EU-countries and between EU and third countries. If you are not one, just jump further to the buying suggestions on the next pages.
1. You order the goods in another European Union country = from
to 
When you and your vendor are both within the European Union it is extremely simple. You pay the price of the goods plus shipment, including 'his' VAT, and they are delivered to your doorstep without any formalities or extra cost, just as had you lived in that country.
2. You order the goods in a 3rd country = From
to 
All non-EU countries are 3rd countries. This means extra cost.
Customs duty is to pay when the price of the goods is
exceeding 150 EUR or the equivalent in other currencies. For a kilt and accessories, it is 12% of
the customs value. The customs value is made up of 1) the price paid for the goods, 2) the insurance cost, and 3) the shipping cost.
If VAT is included in the vendor's price, he should deduct it, prior to charging you.
VAT, calculated on customs value plus customs duty. The rate varies within EU from 15% to 26%. You know what it is in your country, of course.
A
customs clearance fee, a fixed cost, no matter the customs value. It varies from country to country and with transporter. You may calculate with about 20 EUR, equalizing 18 GBP. But could be less, could be more. On a £400 kilt it might be neglectable, but on a a pair of kilt socks costing £10, it triples the price!
3. You order the kilt from a Scottish vendor = From
(
) to 
Scotland is most relevant when talking kilts and accessories. UK is now a 3rd country, but with a
free trade agreement. This means, in contrary to other 3rd countries, that
Customs duty never applies - PROVIDED 100% manufactured in UK.
This should be the case with a wool kilt made to your measures in UK. But if you from a Scottish vendor is buying a complete kilt outfit for over 150 EUR equivalent and part of it has been made in Pakistan, customs duty must be paid for the non-British part of it. The vendor is to fill out a declaration telling to which extent the goods are not 100% British. Most complicated.
Other conditions are the same as for 3rd countries.
The 20% British VAT should be deducted before paying. Some Scottish vendors have prices ex VAT. A few, like Clan by Scotweb, automatically will find out in which country you are living. The price you see is accordingly what you have to pay, and sometimes even in your own currency. Depending on the VAT rate of the individual EU country, VAT is more or less neutral.
Really annoying, compared to earlier, is the customs clearance fee, 'forcing' you to think in bulk-buying, like five or ten kilt socks at a time.

A £60 Persevere kilt from Kilt Society. Before Brexit it was £60 delivered in Germany, after Brexit £77.50 - ex shipment.

On a £200+ kilt in pure new wool the around £18 fix cost declaration fee means less.
Due to the free trade agreament between UK and EU no customs duty applies on this 100% Scottish made kilt in pure new wool. Gunn Ancient tartan.