OK, I took a look at your db. I didn't have time to look at it in depth but I took a look at the form in question. It looks like you used the wizard to create this form, so I'll try to explain this in those terms as best I can. Before we continue, I should point out that you seem to have some minor corruption issues with this form, so I would suggest that you Save after each of these steps along the way, or you may get all the way to the end only to find out that the form won't save.
When you created the form, the wizard created a query for you that is used as the record source of the form. If you open the property sheet for the form, select the data tab and look at the Record Source you will see;
SELECT tblPurchaseOrder. blah blah blah
If you click the elipse (...) to the right of this line it will open the design grid for this query. In the top section of the design grid there are two tables, tblVendors and tblPurchaseOrders. In tblPurchaseOrders double click on the fields Shipping Charge and ShipperID. This will add those two fields to your record source. Then close the query designer. When it asks you if you want to save the changes and update the property click Yes.
Next, add a new text box to your form. Open the properties sheet of the this text box, select the data tab and set the Control Source of this text box to Shipping Charge. Now, add a combo box to your form. When you do this, the combo box wizard should start. In the wizard, do the following;
1) Select "I want to look up the values in a table or query"
2) On the next page select tblShipper.
3) Next, add all of the fields from tblShipper to the Selected Fields list.
4) On the next page select Shipper Name in the first sort box.
5) The next page is where you choose which columns you want to display in your combo box. Make sure that the Hide Key Column box is checked, then take all of the columns (address, city, etc.) except for the Name column and drag the width down to zero, so that the only column you see is the Name column.
6) On the next page select "Store the value in this field:" and select the ShipperID field from the drop down.
7) On the last page put cboShipper in the name box, then click finish (if you don't name it cboShipper then the following steps won't work).
Now add two more text boxes to your form right under the combo box. In the Control Source of the first text box put;
=cboShipper.Column(2)
in the Control Source of the second text box put;
=cboShipper.Column(3) & ", " & cboShipper.Column(4) & " " & cboShipper.Column(5)
Now when you select a shipper from the combo box it should display the Address and the City, State Zip in those two combo boxes.
The reason for doing it this way is that the only Shipper related field available in your form's Record Source is the ShipperID (which is how it should be). All the other Shipper related fields (Name, Address, etc.) are simply displayed on the form via the combo box. Those other fields are not (and should not be) stored in this form's Record Source. Only the ShipperID is. So when you make a selection in the combo box, it stores the selected ShipperID in tblPurchaseOrders and displays all the related shipper info on the form.