Thanks for checking in with us, Dony.

If you'll want to delete the custom fields in an invoice, then that option is unavailable. However, you can remove them from your invoice. 

Here's how:

1. Open or create an invoice.
2. Tick the Gear icon beside the Help menu.
3. Uncheck the custom fields you want to remove in your invoice.

If you need to remove custom fields on the other sales forms, we can go to the Custom form styles page.

Here's how:

1. Go to the Gear icon on the top menu.
2. Choose Custom form styles under Your Company.
3. Tick Edit on the form template you want to change.
4. Tap Content.
5. Select the Header part in the sample form.
6. Under Display, click the Custom field link.
7. Remove the checkmark on it.
8. Press Done. 

For more details about customizing your sales forms, check out this article: Customize invoices, estimates, and sales receipts in QuickBooks Online.

I have a link here that provides you with articles about managing your customer transactions: https://quickbooks.intuit.com/learn-support/en-us/sales-and-customers/07?product=QuickBooks%20Online.

Fill me in if you need a hand with creating recurring transactions. I'll be here to help. Have a great day ahead.