An invoice is sent before payment to request what's owed. A receipt is sent after payment to confirm the transaction is complete. Same transaction, two documents, opposite ends. This article covers how each one is used, what they should include, and common mistakes that confuse clients or delay payment.
In retail it effectively is. For service businesses, it's better to keep them separate so the paper trail is clear.
Legally, in most jurisdictions, only if the client requests one. Practically, sending a receipt is good business hygiene and takes 30 seconds.
Almost nothing in everyday usage. 'Bill' is the casual term, 'invoice' is the more formal one.
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