However to answer your question 'Why are you charged full price for cancelling an Uber Eats order, even if seconds after placing? ', this has to do with the agreement between Uber Eats and the merchants they partner with, which is the restaurant or cafe you're ordering from.
When can you cancel Uber without being charged?
if you cancel in the first two minutes, Uber will generally not charge a cancellation fee. If you cancel after two minutes they generally will charge you, unless the driver is delayed & takes 5+ minutes longer to get to you than the initial estimate.
How do I cancel a just eat order that takes too long?
For delivery and collection orders, head to the order details page, where you'll see the option to cancel your order under 'Can we help with anything? ' Will I be charged for cancelling my order? If you cancel your order, you may be charged an amount up to the full order value.