There is no such thing as an open invoice, an invoice when saved is final and post to the accounts

but yes you could add a charge to it and resend it to the customer – might confuse him though

use an estimate to accumulate charges, then create an invoice from the […]

Resolution for 'accumulate charges issue: 'Is there a way to add additional timed activity to an existing open invoice?' available: Yes.

The accumulate charges 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 accumulate charges issue yourself by looking for a resolution described in the article. If it is a complex accumulate charges issue or you are unable to solve the issue, you may contact us by clicking here or by using other accumulate charges support options.

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