Purchase by tender is optional
Businesses can and do by from each other on many basis - they may pay a list price, they may place an order without knowing the price, they may ask for quotations (from one or many suppliers), they may negotiate a price, they may hold an auction, they may hold a formal tender process.
There is no law that says which they may use in what circumstances. If they want to have a formal tender for the supply on 1 paperclip, they can. If they want to order 78 A380 Airbuses without agreeing an upfront price, they can.