Why wasn't VAT added to my international order?

[mtilden]

Hello!

I am new to Shopify and recently set up a clothing brand. We are based in the US and ship to the UK and Europe. Recently, a UK customer ordered a few items but VAT wasn't added automatically (though it has been added automatically for other orders of lesser value). Once the order made it to the UK, UPS alerted me that I owe a sizable import fee.

My question is two-fold then: why VAT wasn't added for this one international order and if that might be why I have to pay this import fee now. Thanks for any help!