I understand the importance of turning off the Sales tax feature and importing your invoices, smr.

It is now possible to turn off the Sales Tax feature in QuickBooks Online. However, please note that if you have any sales tax transactions, we'll have to delete them. Beforehand, make sure to take note of all the details of these invoices before deleting them.

After deleting the transactions, I suggest performing these steps to turn off the feature:

Automated Sales Tax experience:

1. Go to Taxes, then select Sales tax.
2. Select Sales Tax Settings.
3. Select Turn off sales tax.
4. Select Yes to confirm.

Manual Sales Tax experience:

1. Go to Taxes, then select Sales tax
2. If you see the option to go to the new sales tax experience, click Do it later.
3. Under Related Tasks, click Edit sales tax settings.
4. Under Do you charge sales tax?, select No if you don't charge sales tax.
5. Select Save.

For more details, please see this article: Turn off Sales Tax in QuickBooks Online.

Once done, you can now re-enter the transactions. 

Lastly, I've added this article to help you import multiple invoices in one go:  Upload Transactions in QuickBooks Online.

I'm still all ears if you need further assistance importing transactions, running reports, etc. Just add your reply below, and I'll circle back to help you.