Payment Receipt in Sales Workflow

Hello,

My sales workflow (pre ERPnext) was the following:

  1. Quotation
  2. Sales Invoice
  3. Delivery Note
  4. Payment Receipt
  • Question 1:

Since in ERPnext there are no payment receipts, which document would you send the customer once you have recieved their payment?

  • Question 2:

I can’t tell the difference between the Sales Order and the Sales Invoice? Are they interchangeable?How do they both fit in a workflow such as mine?

Thank you.

Just bumping this up.
Cheers