It actually isn't. An agreement with consideration is a contract. (Consideration means something of value is exchanged.) However the agreement is made. It c

It actually isn't. An agreement with consideration is a contract. (Consideration means something of value is exchanged.) However the agreement is made. It could be on the phone, text, smoke signals, anything. Though if it's denied later you may have a hard time proving the contract was made. Usually an invoice is sent after an agreement. You have to pay those because you already made a contract to pay (written or not, formal or informal), and quite often an invoice provides formality that was lacking before, crystallising the expected amount. The act of paying it generally signifies final agreement to that amount. For business contracts, often an invoice is the first time the buyer sees the exact amount the seller is expecting, because of details that weren't hashed out in advance (no need). For example per-hour freelancing can be like that; the buyer learns the number of hours the seller claims to have worked from the invoice. But people dispute invoices all the time if they aren't expected or differ significantly from the amount expected.