Ron,
You can approach this in two different ways. However I believe this solution would be the best approach, as you could come back in a future date and update the due amounts if needed.
1st I’m assuming you already have two tables
1. A table containing your Customer information.
2. A table containing your Membership information Tierlevel & TierAmt. If this table contains an ID field please delete it in design view.
On your form based off your Customer table,
1. With the wizard turned on, add a combo box to your form
2. From the wizard choose “I want the combo box to look up the values in a table or query”.
3. Select next, and then pick the table that contains your Membership information.  
4. Select next, then using the >> button; add your two fields Tierlevel & TierAmt to the “Selected Fields” window.  
5. Select next, in this area choose how you want the data in the drop down window to be displayed.  I suggest picking ascending on the Tierlevel field.  
6. Select next and you get a quick view of what the data sort will look like.  
7. Select next, now this is one of the important parts. Here you have to decide what “value” you want stored in your table. IE Tierlevel =Associate or TierAmt= $65.00.  
8. Select next, here we choose the option “Store that value in this field” and from the drop down choose the field in your table where you want to store the data.
Now this last part is optional, on the property sheet of the combo box change the value of the “Column Widths” to 1”;0” . this will hide the 2nd column displayed in the combo box.
I also suggest watching this video in the tips & tricks sections “Microsoft Access Relational Combo Box”
Hope this helps….
Gary