What is the ruling on money that must be paid to the hospital when using a private insurance card for the mother that belongs to a usurious bank, and can the amount be given as charity? And must the excess money be returned to the seller if there is a strong suspicion, based on غالبة الظن (preponderance of belief), of a slight excess? And what is the ruling on the extra piece of sweets that was not requested?

The ruling on using someone else's insurance card has been detailed previously. Whatever was gained unjustly must be returned to the insurance company. Donating it as charity is not sufficient, except when it is impossible to reach the rightful owners.

As for the seller who returned an excess amount, you must return the surplus to him or seek his permission to keep it if you are certain or highly confident about it; otherwise, not.

As for the sweets and similar items given by the seller, they are lawful for you if they are a gift from him. If it was a mistake, then its ruling is the same as the previous mistake.