Is it permissible to take the price difference (50 Riyals) or credit card points (10 Riyals) when booking airline tickets for others, while concealing the discount from them, and if it is impermissible, is it permissible to donate it to charities on their behalf without their knowledge?

Taking commission or a discount for yourself without your client's knowledge is forbidden, because an agent is a trustee and is not allowed to profit except with the permission of his client. You must repent to Allah and return the money to its owners. Charity is not sufficient except in cases of despair of reaching the rightful owners or their heirs. So, hasten to return the money or seek their forgiveness before a day when neither a dinar nor a dirham will be of any avail.