You get to the last step of the checkout process, only to discover some unexpected charges have appeared, e.g. delivery charges, tax, etc.
Proflowers is a flower retailer in the United States. They provide a perfect example of the hidden costs dark pattern. This is their home page:
When you add an item to your cart and proceed to the cart page, you see the price is exactly as expected. No hint of any hidden charges.
So you go through the 6-step checkout process. In the 5th step, you fill in your credit card details
Finally when you get to the "review order" page - having entered the delivery name, address & phone number, your credit card details, and your own name, address, phone number, credit card details - you're finally shown the hidden costs. $14.99 delivery and $2.99 "care and handling".
By the time the hidden costs are revealed, you've already invested a great deal of effort and you're likely to feel fatigued. This is probably by design, so you'll be more willing to complete the purchase rather than have to give up and start all over again with another website.