Yours appears to a company from hospitality industry. If you want warning letter to be given for the violation of the laid process then first enumerate what the process is, then write the specific instance i.e. date and time of the default and how the procedure was violated etc. Later give the warning.
Nevertheless, this appears to be a serious matter. Serving food to the guests without uploading the software is bypassing the system. How the lapse was discovered? The value of the transaction could be small but the lapse casts light on the integrity of the person. Serving a warning letter is as good as condonation of the lapse that could have deprived revenue to your organisation. Therefore, you may review your decision of issue of warning letter.