They are going to email you an invoice, and you are required to pay that like any other bill that comes in.
10.6 The Customer shall be billed electronically for the goods and services ordered, and invoices shall be sent to the eMail address he
or she has specified. If the Customer also wants paper invoices to be
sent, Springer may charge a separate fee for this.
My speculation is that they offer these terms because buyers of their products might have to submit a paper bill for reimbursement or submit the invoice to a accounts payable department. You can use the service and it just delays by a few days when you need to settle up.
They will send you an invoice. On the bottom there will be a portion to detach and mail back. They might even include an envelope. You write a check addressed to the payable name on the invoice and mail it, along with detachable part of the invoice to the address they specify.