Using a Santander VISA debit card (GBP) to pay in USD, How much will this charge if my order was $50.50 with free int. Shipping?

I have just made a purchase off the colourpop website
-which is in USD- But I paid it in GBP and needed £50 in my bank. Does the bank exchange that? Since $50.50 Is roughly £36.44 will I be keeping what was left in the exchange or not? I don't quite get how I can pay for something in $ using £ 😂😂 Someone help!?
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