How to restore a deleted customers? No gear icon or "show inactive" from customers page (since we don't have customers otherwise). Please help?

We can restore your deleted customers by making them active again, Fobian. Rest assured, I’m here to help you locate those inactive customers.  

To locate those inactive customers, we can go to the Customer page. Allow me to guide you through the process:

1. Go to the Customer and Leads section on the left panel.
2. Hover to the Customers page.
3. Click the Gear icon, then scroll down and locate the Include Inactive.
4. Once located, tick its box to include it in your customer list.

I've added this screenshot for your visual reference: 

Once done, please check out this article to learn how to delete those invoices: Void or delete transactions in QuickBooks Online.

Alternatively, consider matching the manually created invoice to the downloaded transactions from Airbnb to avoid duplicates.

Also, you might want to utilize the bank rules feature to categorize the transactions from your bank automatically.

Moreover, did you know that you can schedule a free consultation with our QuickBooks Live Expert Assisted? These experts can provide personalized guidance on managing your QuickBooks Online account, focusing on efficient customer management and invoice handling.

Should any issues persist or if you have more questions after trying these steps, feel free to get in touch. We're here to ensure that your QuickBooks setup meets your business needs effectively, Fobian.