1. Apologize: First of all, you need to apologize to the customer. Tell them that this is a mistake, that you have realized the problem and will solve it as soon as possible.

2. Refund the overcharged freight: calculate the overcharged freight amount immediately and refund the money to the customer. You can transfer money through the bank or deduct money directly from their order.

3. Prevent similar mistakes from happening again: check your calculation process, find out the cause of the mistake, and take measures to prevent similar mistakes from happening again. For example, you can redesign your calculation process or install an automatic checking system to prevent similar mistakes.

4. Provide additional services or offers: In order to compensate customers for their inconvenience, you can provide some additional services or offers. For example, you can offer a free speed increase or some discounts or coupons.

5. Follow-up: After the problem is solved, follow up with customers to see if they are satisfied with your solution. If they are still not satisfied, you need to keep looking for ways to solve the problem.

Remember, the key to dealing with this situation is honesty, transparency and responsibility. Let customers know that you care about their satisfaction, and you will try your best to solve their problems.