How do I create same invoices for the past 18 months,

Hi devregy,

Welcome to the community! Creating invoices has never been easier with QuickBooks Online. I'll be happy to assist you with this.

When creating an invoice in the program, you have the flexibility to select any date you want before saving your transaction. You also have the option to copy an invoice to quickly create a duplicate form without having to re-enter everything from scratch. Here's how.

1. Select + New and then Invoice.
2. From the Customer ▼ dropdown, select a customer. Make sure all of their info is correct, especially their email address.
3. Review the Invoice date. Then from the Terms ▼ dropdown, select the due date.
4. From the Product/Service column, select a product or service.
5. Enter a quantity, rate, and change amount if needed.
6. Select a Tax rate.
7. Click on Save.
8. Select More, then Copy if needed to create a new invoice with the same information.
9. Edit where necessary, and Save.

For more on how to manage invoices in the program, check out this article: Create invoices in QuickBooks Online

I hope this helps! Have a great day. 

@devregy 

You may prepare the Invoice data in XLS and utilize the trial period of an importer tool

https://transactionpro.grsm.io/qbo