QB Issue Resolution:

you will have to either type that info into a blank description block – this works in desktop I have not tried it in QBO

or gear>settings>company settings
sales>customize look and feel button

scroll down and use a custom message box in the footer section

Resolution for Issue 'I want to add additional info on invoice about late fee and interest if not paid in 30 days.' available: Yes (Solved).
Source: Intuit Community forum.

I want to add additional info on invoice about late fee and interest if not paid in 30 days.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Invoice
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Error Recovery for Invoice [Canada]
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