vatcalc.onl

I just ordered something from a UK-based company, and it has already been delivered. Now they want me to pay VAT separately. I thought the pricing already included VAT because there was no indication of any extra fees at checkout. This request made little sense to me because UK legislation says that all prices offered to customers must include VAT directly. I used vatcalc.onl, a free tool that helps with UK VAT calculations, to see how VAT works by changing the price of an item by adding or taking it away.