Amazon charges them money and the goods are in a UK warehouse presumably, so they will have bulk imported them from the EU (thank you again David, Nigel, Theresa and Boris, really can't thank these greatest of politicians Britain's come up with enough) and paid HMRC some money for the favour of allowing the goods into the country.
Likely the website in LT won't charge you tax, and the Latvian packaging employee will be paid considerably less compared to his Amazon UK counterpart.
Mostly it'll be Amazon's listing and payment processing fees that drives the price up.