I've been thoroughly enjoying our collaboration and the efficiency brings to our invoicing process.

To further enhance our experience, I'd like to suggest the incorporation of a digital signature functionality for invoices.

This feature could enable us to:

1) Send Invoices for Digital Signatures: Streamline the approval process by allowing digital signatures directly on the invoice to collect signatures from client.

2) Real-time Tracking: Gain transparency by tracking the status of the invoice, knowing when it's viewed and signed.

3) Centralized Storage: Maintain a secure, centralized repository of digitally signed invoices for easy retrieval.