I understand that you need digitally signed invoices. Let me help you.
QuickBooks Online doesn't support DSC yet. You'll want to look for a third-party app that you can integrate into QuickBooks Online. You can visit and search for one in the Apps tab.
Also, another thing I can suggest for now is to create your own invoice template using a .docx file. From there, add your signature or stamp. Then, upload it to QuickBooks Online. You can find the detailed steps here: Import custom form styles for invoices.
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