Point of sale – merchant guide

With the KAI-Pay point of sale you accept Bitcoin payments via Lightning, for example at the counter or a market stall. The point of sale is a separate page (kasse.kai-pay.net) without keys: it can only create invoices and can never send money. Every payment goes straight into your self-custodial KAI-Pay wallet. There you see the daily and monthly closing and can issue refunds.

KAI-Pay is a public beta. The point of sale is a payment terminal, not a cash register (see Limits).

1. Requirements

• A KAI-Pay wallet whose 12 words you have backed up and confirmed in the app.
• A Lightning address: create it in the wallet under “Receive”.
• Internet on the till device. No account or sign-in is needed.

2. Set up the point of sale

1. In the wallet, open the menu and choose “Point of sale & closing”. At the very bottom you find “Set up point of sale”.
2. Optionally enter a till name, for example “Counter 1” or “Market”. The link and QR code change with the name.
3. Open the point of sale:
   • On the same device: “Open the till on this device”. It opens in a new tab, or in the Android app in the browser.
   • On a separate till device (e.g. a tablet at the counter): scan the QR code with that device, or copy or share the link and open it there.

The link contains your Lightning address, the till name and a public receipt key. These details are in the part of the link after “#”; they are not sent to any server. The point of sale stores them on the till device. The next time you open kasse.kai-pay.net it starts directly; you no longer need the link.

If someone opens a link with a different address or a different receipt key on a till that is already set up, the point of sale asks before switching. Only confirm if you set it up yourself. The point of sale and the wallet both show the fingerprint of the receipt key, so you can compare them.

3. Taking payments

1. Enter the amount: in euros (converted with the rate shown) or in sat (“Enter in sats”).
2. “Charge”: the point of sale shows an invoice as a QR code.
3. The customer scans it with any Lightning wallet and pays.
4. The point of sale shows “Paid” and a numbered payment receipt. It is proof of payment, not a till receipt.

Notices from the point of sale:

• Cancel: a cancelled invoice can still be paid. If the payment arrives later, the point of sale says so explicitly.
• Page reloaded: “Open invoice resumed” – the point of sale keeps checking the running invoice. Do not charge twice.
• “Invoice expired – not paid”: charge again.
• “Payment status unknown” (no connection): check the incoming payment in the wallet before charging again.
• “Recent invoices” lists the latest invoices of this device (at most 20 from the last 7 days). The wallet’s history is always what counts.

4. Till name and several tills

The wallet does not store the till name. It travels like this:

1. The name is in the link. The point of sale stores it on the till device, together with a random device ID and a running receipt counter.
2. With every payment the point of sale sends an encrypted note: device ID, till name, receipt number, euro amount and rate. Only your wallet can read it.
3. The wallet reads these notes from the payments and assigns them to the tills in the closing.

• Renaming: open the link with the new name on the same till device. Device ID and receipt numbers continue; earlier payments keep the name they had at the time.
• Without a name: the point of sale keeps its previous name, or is called “Point of sale”.
• Several tills: set up each device with its own name. Two devices with the same name appear as separate rows in the closing, and the wallet points this out.

5. Daily closing

In the wallet, under “Point of sale & closing”, choose a day. For each till you see:

• the number of payments and the total in sat,
• the euro amount according to the till (at the rate at the time of payment),
• the range of receipt numbers as well as missing or duplicate numbers,
• refunds and the net amount.

In addition, the wallet lists “Other incoming” (payments to your address that did not come through the point of sale) and payments still pending.

6. Monthly closing and CSV export

“Export day as CSV” or “Export month as CSV” saves a file with one row per transaction:

• date, time, till and receipt number,
• amount in sat and fee,
• euro amount according to the till and rate,
• payment ID and type (sale or refund, refunds negative).

The totals in sat and euros are at the end.

The wallet keeps no separate cash book for this. It calculates the closing from the payment history each time. You can therefore export any past month again, as long as the payments are in the history. The wallet derives the key for the receipt notes from your 12 words; the notes stay readable after a restore as well.

The point-of-sale export only contains payments made through the point of sale. For an overview of all income, also export the wallet history as CSV; it contains every payment.

7. Refunds

1. In the wallet, open the point-of-sale payment in the history and choose “Refund”.
2. Enter the customer’s Lightning address or invoice. You can refund at most the amount still open.
3. Check and confirm. The refund appears in the closing as a negative amount. For Lightning addresses that accept a note, the wallet adds “Refund for receipt …”.

The link “refund for receipt” is stored only on the device you refunded from. If a refund is still on its way, the wallet warns before a second one.

8. Security and data

• Staff devices: the point of sale has no access to your money and cannot send. Words, PIN and funds stay in your wallet.
• What others see: the receipt note is encrypted. The payment service still sees the amount, time and your Lightning address. Encryption does not mean anonymity. Details are in the privacy policy.
• Shared devices: point-of-sale data (address, till name, receipt counter, recent invoices) is stored in the browser of the till device. On someone else’s or shared devices, remove it via “Other address” or the browser data.

9. Limits

• Not a cash register: the point of sale is not an electronic recording system with a certified technical security device (TSE) under the German Cash Register Security Ordinance (KassenSichV). Whether a cash register obligation applies to you depends on your actual business. You fulfil your own recording, receipt and tax obligations (terms of use).
• Overview, not accounting: the closing and CSV are a payment overview for you. They contain no VAT breakdown and no cash report; you can use them as a basis for your bookkeeping.
• Beta: features may change. Save monthly exports regularly. We have not yet fully verified that all receipt notes come back after switching devices.

Questions? FAQ or kontakt@kai-pay.net.