To add or update your billing and shipping address on a Purchase Order:

If you want to update a saved PO, first click one purchase order. If you are creating a new PO, skip the first two steps.

Then, click on Edit. 

If you are creating a new PO, start here.
Click on Switch to Advanced Mode in the upper right corner. 

You'll then see fields for billing and shipping addresses.
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Once you enter this information, it is stored in Inventory Planner so you will not need to enter that each time you create a Purchase Order.