You can send invoices directly to your customers from TradeGecko.
Go to the sales order. When the order is Finalized, you are presented with an option to either create a manual invoice (this is a multiple invoicing feature) or to automatically create one.
Click 'Invoice All' and the invoice is generated.
Click on the 'Email' button on the top of the page.
You can edit the email accordingly and email it to yourself using the cc field.
Click 'Send' and your customer will receive an email of the invoice.