How can I add a custom column in invoice and sales receipt i need to add Units (Sqft or Cuft or Lft)

You've come to the right place, @punitha-qb.

In addition to Rustler, you can consider adding a custom field to your invoice and sales receipt. This way, you'll still be able to add the unit (Sqft, Cuft, or Lft) to the invoice. Let's go to your Account and Settings and add a custom field from there. I'd be glad to guide you how.

In your QuickBooks Online (QBO) account:

1. Go to the Gear icon.
2. Select Account and Settings.
3. Choose the Sales menu.
4. Select the Sales form content.
5. Put a checkmark on the Custom fields section. 
6. Click Save, and hit Done. 

Here's an article you can use as reference that can guide you on how to personalize your invoice: Customize invoices, estimates, and sales receipts in QuickBooks Online.

I'm also adding this useful article that can help you view and send to your customer their balances: Create and view customer statements.

Please know that you're always welcome to post here anytime you need help. I'm more than happy to work with you. Have a great day and more success in your business!