Basically, enable voucher sales for appointment booking services that send a Voucher in PDF with a QR code and reception has option to scan the QR code and validate the ticket and mark it used.
These plugins have this, but they are clunky and have compatibility issues with FluentSMTP and PostSMTP.
https://woocommerce.com/products/pdf-product-vouchers/
https://flexiblecoupons.net/products/flexible-coupons-pro-tickets-add-on/