How can money be returned to the postal service after using recycled stamps, especially given the unknown total amount, the current lack of funds, and the change in residence?

What you did is considered reprehensible embezzlement and deception, and consumption of others' wealth unjustly. Allah has forbidden this, saying: "And do not consume your properties among yourselves unjustly." The obligation is to repent to Allah and return this money to its rightful owners (the Postal Authority). If you are unaware of the exact amount, then estimate it based on your best judgment and pay what you predominantly believe will clear your liability. Exert your utmost effort to identify the owners of the money. If you have spent the money and do not possess what to pay back, the amount remains a debt upon you until you are able to repay it, just like any other debt. If you are unable to repay, then there is nothing upon you, as Allah the Almighty said: "So fear Allah as much as you are able." And if one dies in poverty, it is hoped that Allah will settle his debts on his behalf with his claimants on the Day of Judgment.