How can I change the default amount that appears in the "rate" field when I create a new invoice or sales receipt ?

Welcome back to the Community, @galvanicmac.

Changing the default rate that appears when creating an invoice or sales receipt in QuickBooks Online (QBO) is pretty easy. This can be done by editing the item's rate in the Products and Services page.

Here's how:

1. In QBO, go to the Sales menu at the left pane to get to the Products and Services page.
2. Locate the item that you'll use in an invoice/sales receipt.
3. Click the Edit option under the Action column. 
4. Change the rate in the Sales price/rate field.
5. Hit Save and close to complete the process. 

Once done, create a test invoice or sales receipt using that item to double-check.

Just in case, I'll add these articles for future reference:

• Edit a product or service.
• Change Product and Service types.

I want to make sure everything is taken care of for you, so let me know if you have any other issues or concerns by leaving a comment. I'm always happy to help. Have a great weekend!