A macOS 10.16 user recently reported that ZKAPs are not being redeemed. The Gridsync debug log suggested that this was due to TLS certificate verification failures:

2022-02-21T15:25:33.795759+00:00 DEBUG line_received [<Filtered:GatewayName:1>] >>> 2022-02-21T15:25:33+0000 [_zkapauthorizer.controller.PaymentController#error] Redeeming random tokens for a voucher (b'XBpH0ykPlR3fIWmHJGHdSgpFfNI95VsE-GPid3BzfARK') failed: RequestTransmissionFailed([<twisted.python.failure.Failure OpenSSL.SSL.Error: [('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')]>])

This problem had been observed previously on Windows and using certifi's CA bundle was sufficient to fix it. After speaking with the macOS user and having them test a build that employs the same solution, they confirmed that the ZKAPs redeemed successfully with certifi's CA bundle.