Does anyone know if it's possible to set the Due Date of an invoice to be the same on all invoices, instead of manually updating them, one by one by one....?

I'll help you set up a new term to ensure you have the same due date on all invoices, Alwyn.

Yes, we can set a specific term for all your invoices. It's important to note that the term we'll be creating will be applied to your upcoming invoices. 

Here's how you can do it in your QuickBooks Online (QBO) account: 

1.  Go to the Gear icon. 
2. Click All lists, then Terms.
   
   
    
3. Select New, then fill out the necessary details to your preference. (Example: Due by a certain day of the month, then input the date you want to set to apply them on all your invoices.)
   
   
    
4. Once done, hit Save. 

Then when you create a new invoice, ensure to select the created term. 

However, if you want to modify your prior invoices' due dates, you'll have to manually edit them: 

1. Go to the Sales menu, then click Invoices. 
2. Find the invoice you wish to edit, then select Edit invoice. 
3. On the invoice page, go to the Terms, and set it with the new one. 
4. Once done, choose Save or Save and close. 

Moreover, you can customise and add specifics to your sales forms in QBO. 

I'd be more than happy to assist if you require further assistance managing your invoices in QBO. Have a great day ahead of you.