If your purchase is £135 or less (and the seller has registered for UK VAT) the VAT is paid at point of purchase and there will be no duty and Royal Mail won't charge.
If it's over £135 it'll be the same as it always was; cost of goods plus shipping and insurance for duty calculation, VAT calculated on that total, handling charge on top.
https://www.simplyduty.com/import-calculator/