As services embedded into public spaces become increasingly transparent, one peripheral aspect of use continues to demand explicit user attention: payment. UbiPay is a system that carries out small everyday payments in a way that minimises user involvement by choosing an interaction method based on context information. The aim is to make paying like breathing: something we are only peripherally aware of unless we exert our resources beyond the usual. This idea has powerful implications for business and design. Keywords Electronic payment, mobile devices, transaction costs ACM Classification Keywords K.4.4 [Computers and Society]: Electronic Commerce ---Payment Schemes, Cybercash, digital cash; H.5.2 [Information Interfaces and Presentation]: User Interfaces---Input devices and strategies