Is it permissible to increase the price of a commodity beyond the price set by the merchant without the buyer's knowledge, given that the merchant is informed of this increase, and that the announcement is made through private accounts and the one who benefits from the increase is the announcer?

If you are not an agent for the buyer, then the permissibility of increasing the price depends on the merchant's knowledge and consent, because he is the owner of the commodity and you receive your fee from him. As for the customer, what matters in his case is his satisfaction with the price, regardless of its details.