How can I be sure that my invoices are sent, received and processed by my client?

After uploading an invoice, it is assigned a status. Each invoice processed on SY has a status that gives information on it processing progress. To view the status list enad definition, go to the FAQ  "What are the processing statuses”

To track the invoices sent to your customers and their progress, click on "Invoices" on the home page or in the menu on "Invoices" / "Sent invoices".