It is asking to subscribe if we are sending invoice to specific customer from Sales option. However it is not working from for other customer accounts. Please help.

You've come to the right place for an answer, @worldstardelhi.

I'll ensure that you can send customer invoices seamlessly in QuickBooks Online. I have various troubleshooting steps here to help resolve the subscription pop-up issue in the Sales page.  

To start with, ensure that your customer payment method has no credit card information to sort this out. Let me show you how to check:

1. Go to the Sales tab on the left side, then select Customers. 
2. Choose the customer's name in the list and click the Edit tab. 
3. Pick the Payment and billing button and review the details. 
4. If there are information, please remove them and choose other payment option in the box.
5. Click Save. 

Once done, re-send the invoice to verify if it's performing well. If the issue continues, I'd recommend re-creating the customer details to retrigger the process.

Here's how: 

1. Go to the Sales tab, then choose Customers. 
2. Click the New customer button and enter the basic information in the fields. 
3. Pick Payment and billing, then select either Cash or Check from the Payment method drop-down. 
4. Hit Save.

Once done, re-generate the invoice and send it again to that specific customer to verify if it's already working adequately. If QuickBooks no longer asks you to subscribe, I'd suggest merging your old and new customer. This way, the information and transactions will be gathered. 

Here's how: 

1. Go back to the Sales menu and select the Customers tab.
2. Select and open the old customer profile.
3. Edit the name and pick a Display name in the box. 
4. Hit Yes, then Save. 

For additional insights, see this link: Merge duplicate customers.

Additionally, check out these articles below on how to combine multiple transactions into a single record and other related matters: 

• Record and make bank deposits in QuickBooks Online
• Sales and customers

You can always notify me if you have any other QuickBooks questions. I'm a few clicks away to help. Keep safe, worldstardelhi.