Can I cancel an e-Invoice from Aicountly Billing, and what if more than 24 hours have passed?

Yes, while the e-Invoice is still active, Billing's e-Invoice panel on the bill has a Cancel button. It opens a Cancel this e-Invoice on the portal? panel where you choose a Reason the portal records (the portal will not take a cancellation without one) and can add an optional remark. Confirm with Cancel this e-Invoice, or press Keep it to leave it as it is.

The portal takes a cancellation only within 24 hours of the e-Invoice's acknowledgement, and the panel tells you how much of that window is left. A cancelled e-Invoice cannot be revived: the bill has to be raised again, with a new IRN.

If more than 24 hours have passed, the portal will most likely refuse the cancellation, and the e-Invoice is undone with a credit note instead. Since the right treatment depends on your invoice and return period, confirm it with your CA before you issue it.