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Though having cash is not enough. The stores also need to be able to accept cash without internet usage. I think we had a case in germany a few years ago, where some supermarkets could not sell anything, because the servers, to which the local payment system connected (also uses for cash) didn’t work. Not sure, if that was because of a security incident.
The OP doesn’t say that they have a trademark. For me it sound as they don’t have one. Only a registered domain and the name of the software on github. The letter came on the grounds of the oponnents trademark.