https://qrapi.io/
QR Code API | Generate Custom Static Codes via Rest API | Trycon
With Trycon's QR Code API, create upto 1 Million QR Codes programatically via your website or information system. First 20 Requests are free!
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https://developers.google.com/wallet/tickets/transit-passes/qr-code/resources/faq
Google Wallet API FAQ | Transit QR code | Google for Developers
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