Hello AndreiD,
Thank you for your request.
When you pay for a Half-Fare travelcard online on sbb.ch, you order a new Half-Fare travelcard. Invoices from existing contracts such as the one you already had cannot be paid on sbb.ch.
Or you can also find your invoices at www.swisspass.ch when you log in.
If you lose an invoice, you can request a new one by phone or contact form: Overview | SBB help & contact
You can pay the bill later either by post, e-banking, bank, at an SBB counter or by calling us.
Have a nice day,
Regards MelanieD