NFC transactions become very popular as they limit the time needed for a user to pay. The advantage of this technology is often seen by considering the user's point of view. In this paper, we address the use of NFC transactions for the merchant side. We propose different architectures in order to use a mobile phone as a point of sale. We compare different solutions considering some criteria such as the security assessment which is a major concern for e-payment or the facility of integration. We propose an original approach to authenticate users through the use of the touch screen of the mobile phone. The biometric authentication is based on signature dynamics (match-on-card approach) achieving a usable solution for this application.