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.
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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.