Is it permissible to return an old debt to the grocery store owner via an intermediary, informing him that the amount is from me, and is it necessary to return the debt to each individual separately in the same grocery store?

Financial rights must be returned to their owners directly, or sent with a trusted person after ensuring delivery. What was taken from the grocery store must be returned to its original owner, not to the sellers in it, as they are merely agents. It is preferable to go in person to resolve the matter.