The checkout_id is the foreign key. A single checkout creates a payment event that may contain several payment orders.

When we call a third-party PSP to deduct money from the buyer's credit card, the money is not directly transferred to the seller. Instead, the money is transferred to the e-commerce website’s bank account. This process is called pay-in. When the pay-out condition is satisfied, such as when the products are delivered, the seller initiates a pay-out. Only then is the money transferred from the e-commerce website’s bank account to the seller's bank account. Therefore, during the pay-in flow, we only need the buyer’s card information, not the seller’s bank account information.