Many a times this happens that you will be asked to send an Invoice to your member before he/she wires the money to you..
For this reason , a new option is provided for an existing member.
Search for the member and click on the action Icon in the last column. The Member details popup comes up. In that there is a Invoice Tab. Select the Manage button under this tab and you will see an option to Generate Invoice. Click this call to action and the invoice is generated. A new row would be created and an option to download that invoice.