Invoices are generally sent for products or services before or after fulfillment. For example, a web designer or software engineer might send an invoice for a new website with 25% as a downpayment and the remainder on completion. This is not a quote or proposal, so it should have only the required information for payment, services rendered, prices, etc. and ideally a link/button to receive payment.  

Tip: An invoice is a nice opportunity to thank a client. Uplifting comments can put clients in a good mood. If you know them in a personal/friendly context, sometimes a meme can help break the ice on making a payment. :)

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NB: An environmental company that provides plant and flower services for offices and other environments, issuing their invoices via an invoicing app.