Building a Bitcoin POS With Merchants, Not For Them — Erik | bitcoin++ Berlin, payments edition

Summary

Erik describes the design and development of Numo, an Android point-of-sale app for accepting Bitcoin through Cashu eCash. His main argument is that Bitcoin payment tools should be designed for both sides of a transaction: the person paying and the merchant receiving payment.

Making Bitcoin payments feel intuitive

Erik contrasts technical arguments for Bitcoin payments—such as transaction speed and censorship resistance—with the less technical experience of using them. Drawing on the original iMac’s handle as an example of design that made technology feel more approachable, he argues that a simple tap-to-pay interaction can make Bitcoin payments feel more tangible and familiar.

Numo supports contactless Cashu payments between phones. The tap experience uses haptic feedback and sound, and the team is working toward compatibility across iOS and Android wallets. Erik says the interaction can make a successful payment feel clear and satisfying, rather than like an abstract change on a screen.

Designing for merchants, not just checkout

Numo’s early design borrowed familiar POS ideas from products such as Square, including adding items and viewing inventory. But Erik says reviewing other POS apps only provides a starting point: merchant testing revealed needs that developers had not anticipated.

The team tested Numo at Bitcoin++ events, placing it in the hands of non-bitcoiners who had to learn the app quickly and use it while serving customers. Feedback led to several practical features:

• Automatic withdrawals: Merchants can set a Lightning address and a balance threshold. When the balance exceeds that threshold—for example, 50,000 sats—the app merges and sends funds to the merchant’s wallet. This offers a way to reduce reliance on the Cashu mint, though merchants still need to understand the custody trade-offs.
• Transaction labeling: A merchant who needed customer-level records for accounting showed the team that payment history alone was insufficient.
• Sales summaries and CSV export: Merchants wanted to see what sold and the day’s total without manually reviewing transactions. CSV export can help connect sales data to existing workflows.
• Accounting integration: Erik sees direct integration with merchants’ accounting software as a valuable future improvement, though he notes it is more complex.

His broader lesson is that the less visible work after payment—record keeping, reporting, and accounting—can be as important to adoption as a smooth checkout.

Open questions and adoption barriers

Cashu makes it possible for a merchant to start accepting payments quickly, without the channel and liquidity management associated with self-custodial Lightning. That convenience comes with a custody trade-off. Erik questions whether onboarding explains the risks clearly enough and whether merchants understand how to withdraw funds to self-custody.

Another obstacle is requiring a merchant to use a separate device or app. Erik says Bitcoin would be easier to adopt if it could become another payment option in the POS system merchants already use.

In response to a question about whether merchants want to hold Bitcoin, Erik offers an anecdotal—not comprehensive—answer: most merchants he spoke with wanted to convert it to fiat soon after receiving it, while one kept a small amount because it was a minor share of their business. He argues that Bitcoin should fit into merchants’ existing payment routines, rather than requiring them to first decide to become Bitcoin holders. He also notes that conversion can raise regulatory and tax questions.

Technical references

Erik invites wallet developers to explore the NUT-18 payment request and the NFC payer specification on GitHub, and to work on cross-wallet tap-to-pay support, speed, and handling edge cases such as a phone being removed before a tap completes.

Reviews or tutorials

This is a product-design field report, not a step-by-step tutorial or formal product review. It describes user testing and lessons from merchant feedback.

Main speakers and sources

• Erik — Presenter and Numo designer/developer.
• René — Referenced for an earlier presentation on payments.
• Merchant testers and an audience questioner — Sources of anecdotal feedback discussed during the talk and Q&A.