This may be useful to you if you need to start a new estimate number sequence in QuickBooks Online. You may want to do so if you experience duplicate QuickBooks transaction numbers that cause issues. 

How To Reset the Estimate Sequence

Step 1: Turn on 'Custom Transaction Numbers' in QuickBooks

Do this by clicking the gear icon on the QuickBooks home page, click 'Accounts and settings' then click 'Sales'. 

Edit the 'Sales from content' section and turn on the Custom Transaction Numbers. Make sure to click the Save button.

Step 2: Create a New Estimate & Set your New Sequence Number

Create a new estimate in QuickBooks (for any customer and item). 

On the estimate, change the number in the 'Estimate no.' field to the starting number of your new number sequence. Make sure you click 'Save and close'.

For example, 5000 if you want to reset the sequence to start with 5000; the next estimate will be 5001, followed by 5002, 5003, etc. 

Optional - Step 3: Turn off 'Custom Transaction Numbers' in QuickBooks

If you don't want to keep Custom Transaction Numbers on, make sure you turn this setting off. Do this by clicking the gear icon on the QuickBooks home page, click 'Accounts and settings' then click 'Sales'. 

Edit the 'Sales from content' section and turn off the Custom Transaction Numbers. Make sure to click the Save button.