No. A sales invoice is a record of a financial transaction created by the business owner and delivered to the customer. A sales order is the opposite: it's created by a customer who has placed and paid for an order. Sales orders are typically prepaid by the customer while sales invoices notify customers of payments due. In other words, a sales order shows what a business owes a customer, and a sales invoice shows what a customer owes a business.
A purchase order (PO) is a document created by a customer and sent to the seller to confirm a pending purchase. Purchase orders include information about which product a customer is ordering, how much of that product they're ordering, when they need the order by, and when they'll pay for the purchase.
The PO should also include any relevant terms and conditions, which vary from company to company. For instance, depending on your company, you might note that the seller isn't responsible for damage that occurs during transit, or that the buyer won't pay for goods that get lost in the mail.
POs are important to inventory management: when customers place purchase orders, you can keep enough inventory in stock to fill their needs without overstocking.
In contrast, sales invoices record a transaction that has already been made—the seller has already sold the product or performed the service, and all that remains is for the buyer to finalize their payment.
No. While an invoice is a request for a customer to pay for a product or service they've already received, a receipt is a confirmation that the customer already made the purchase. Both sales receipts and sales invoices are important documents for business owners to hang on to—they'll help you track your profits and business expenses, not to mention file your taxes correctly at the end of the year.
Unlike a typical sales invoice, a pro forma (or proforma) invoice is created by the seller before they perform any services or sell any goods. They're similar to a quote, proposal, or estimate, and they help you and your customer align your expectations before you get into the actual business transaction.