You are correct that the Goal Seek function in Excel is typically used to find a specific value in a cell by changing the value in another cell. However, it is possible to use Goal Seek to minimize a value by setting it to zero.

Here's how you can use Goal Seek to minimize the total cost of your shipments:

1. First, make sure you have a formula in your spreadsheet that calculates the total cost of a shipment based on the shipment size and storage costs. Let's say this formula is in cell A1.
2. Next, choose the cell where you want to enter the shipment size that will minimize the total cost. Let's say this is cell B1.
3. Enter an initial guess for the shipment size in cell B1.
4. Go to the "Data" tab in the Excel ribbon and click on "What-If Analysis" and then "Goal Seek."
5. In the Goal Seek dialog box, set the "Set cell" field to the cell that contains the total cost formula (A1 in our example).
6. Set the "To value" field to 0, since we want to minimize the total cost.
7. Set the "By changing cell" field to the cell that contains the shipment size (B1 in our example).
8. Click "OK" to run the Goal Seek function.
9. Excel will now try different values for the shipment size until it finds the one that minimizes the total cost. Once it has found a solution, it will display a message asking if you want to keep the solution. Click "OK" to accept the solution.
10. The shipment size in cell B1 will now be set to the value that minimizes the total cost.

I hope this helps you optimize your shipment sizes and storage costs!