You can send invoices without the online payment gateway links by following the steps given below.

Remove the online payment link from the invoice notification emails

• Go to Settings --> Invoice Notification (under "Email Settings").
• Remove the place holder for OnlinePaymentURL. i,e,. remove the following link from the invoice email message content : <a href=%OnlinePaymentURL%>Online payment</a>
• Click the Save button

Skip selecting online payment gateways : While creating an invoice, you'll see an option to select the payment gateway, un-check all the payment gateway options.