I'm having an issue where the invoices that are created by QB are using invoice numbers that already exist. With the mobile app I can't even change the inv nbr.

Thank you for taking the time to post your concern, alext-pobox-com. I understand how inconvenient it is to have duplicate invoice numbers. I'm here to assist you with this.

Let's use the Custom transaction numbers feature to modify your invoice numbers in QuickBooks Online. Before doing so, I'd recommend accessing your account via browser.   

Here's how:

1. Go to the Gear icon and then select Account and settings.
2. Select the Sales tab.
3. On the Sales form content section, turn on Custom transaction numbers.
4. Click Save, then select Done.

For more details, please see this article: Change the Invoice Number on Sales Forms.

Once done, you can now edit the invoice numbers. I'll show you how:

1. Go to Sales and then choose Invoices.
2. Select the No. column to sort it.
3. When the Sales populate, it toggles the numbers in ascending and descending order.
4. Check for duplicates, then click the pencil icon to edit it.
5. On the Invoice no. filed, add the desired number.
6. Once done, click on Save and close.

Additionally, I've included these articles that'll help you personalize your invoices in QuickBooks Online. This way, you'll be able to add the info that matters most to your business:

• Customize Invoices on the QuickBooks Online Mobile App
• Customize Invoices, Estimates, and Sales Receipts on the Web

I'm only a post away if you need more help in completing your other tasks in QuickBooks, alext-pobox-com. It's always my pleasure to help you out again.

Thanks for the reply.  I already have this feature enabled (by default).  The problem is, how do I know which invoice numbers are unused?  I'd have to go and look at all the invoices and find the next available number.  On a phone that is really hard.  Even on a browser it requires stopping what I'm doing and going to look at the existing invoices.  All I want QB to do is automatically pick the next available invoice number.  Is this possible?

I've got you covered, @alext-pobox-com.

Yes, it is possible. You'll just need to turn off the Custom transaction numbers feature so QuickBooks will automatically add the invoice number in sequential order. Though the Invoice no. field is hidden on the invoice page after turning off the feature, it'll automatically pick the following available invoice number. Here's how:

1. Sign in to your QuickBooks Online (QBO) account.
2. Click the Gear icon, then select Account and Settings.
3. Select the Sales menu.
4. Go to the Sales form content section, then turn off the Custom transaction numbers option.
5. Click Save, then Done.

You might also want to read this article: Customize invoices, estimates, and sales receipts in QuickBooks Online. This allows you to personalize and add specific info to your invoice.

Please know that I'm just a reply away if you need any further assistance managing sales forms in QBO. Wishing you all the best, @alext-pobox-com.