I have client that went to a new billing system, I need to delete a ton of invoices created incorrectly..how can I do this without doing it one by one?

Welcome and thanks for joining the Community, @attentiontodetai.

I'm here to share some information about deleting invoices in QuickBooks Online (QBO).

Deleting multiple invoices at once or by batch is a wonderful idea! However, this option is currently unavailable in QBO. You need manually open those invoices and delete them one at a time. Here's how:

1. Go to the Sales menu on the left panel, then pick Invoices.
2. Click the invoice that needs to be deleted, then select More at the bottom and choose Delete.
3. Select Yes.

As a reference, you can also check out this article for further guidance: How to void or delete an invoice or other transactions.

I can see how this feature would be helpful for you and your organization and I encourage you to submit feedback. These requests and suggestions are forwarded to the Product Development Team for consideration in future updates. This can be done directly through your account by clicking the Gear icon in the top right corner, then select Feedback or by going to the following link: QuickBooks Online Feature Requests. I'm going to submit feedback from my side as well.

That should answer your concern for today. Please let me know how it goes or if I can be of additional assistance in the comment section. I'm always here to help you out. Take care and enjoy your weekend.