QB Issue Resolution:

maybe check the market place sometimes third party vendors have apps.

Find a local Pro Advisor that has the Accountant’s Version of your software. they might be able to some batch processing.

Resolution for Issue 'Adding classes to specific customer invoices' available: Yes (Solved).
Source: Intuit Community forum.

Adding classes to specific customer invoices: 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.

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