Why did i receive an email about billing me for an app even if i deleted it in less than an hour?

Just deleting an app obviously won't cancel the purchase of it and therefore the emailed receipt, nor will it cancel a subscription if it had one (to stop an app from auto-renewing : View, change, or cancel your subscriptions - Apple Support)

All sales are considered final in most countries.

If you have received a confirmation email, click the Report a problem link at the bottom.

Purchases problem

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.

iTunes Support

iTunes Support – world wide