Can I add a membership number to the Donor record, that will appear on invoices, how do I do this?

Glad to have you here, @secretary-drtn-o.

You have the option to create a custom field for any additional details on the invoice. Here's how:

1. Click the Gear icon and choose Custom Form Styles.
2. Locate the invoice template then select Edit.
3. Go to the Content tab, and tap the Header section of the template.
4. Use the + Custom field link to expand the tabs.
5. Mark a blank custom field then input a description you want, like Other Invoice Number.

When creating an invoice, enter other number manually. See this sample snip:

See this article for more information about customizing invoices in QBO: Customize invoices, estimates, and sales receipts in QuickBooks Online

You can also use the Class or Location tracking feature. These two features will let you run through account balances by department, business office or location, separate properties, or any other meaningful breakdown of your business, and other segments you want to keep a close eye on.

To turn these features on, follow the steps below:

1. Tap the Gear icon and choose Account and Settings.
2. Go to the Advanced tab and pick Categories.
3. Click the Track classes or Track Locations slider. 
4. Select Save and then Done.
   
5. When creating a new transaction related to the mortgage, just fill in the Class or Location field to include them on your reports. See this sample screenshot, you can also do it on invoices:
   

When you run financial reports, simple include the class or location by clicking the Customize option. See this:

If you have further questions about invoice customizations or need anything else with QuickBooks, let me know by adding a comment below. I'll be right here to help you. Have a great rest of the week.