Follow the steps below to create a fee and then you will be able to preview your invoice.  You can simply remove the asset values to clear the fee when you're done.

To Preview an Invoice (and Confirm Your Customization Looks Good):

Go to the Fees page

Make sure you are looking at the correct billing period

Click on Edit Billable Assets in the pencil menu for the desired client

A slideout window will appear

Enter Market Values and Exclusions for all accounts

Click Save

-You can continue to modify your invoice customization and return to the Fees page to preview the invoice.  When the invoice format looks good, you can delete the market values and exclusions when done so new accurate market values can be supplied when appropriate.