How can we apply the default charge to overdue invoices

I'm happy to guide you as to how you can apply charges to overdue invoices in QuickBooks Online (QBO), @userzunaira.shabbir9. This way, you can manage your income and sales transactions accordingly.

There isn't a need to upgrade your subscription to apply charges to overdue invoices in QBO. You're unable to automatically add a default late fee charge to your invoices. You'll have to create product/service items for late fees. Then, manually add them to your invoices (existing) as a line item (if you haven't sent the invoice yet) or create a new one for the fee and send it to your customer (if you already sent the invoice). To do this, here's how:

1. Go to the Sales menu.
2. Select the Products and services tab.
3. Click New.
4. Choose Service.
5. Give the late fee a name.
6. From the Category drop-down, select the category that best describes the fee. Learn more about item categories.
7. Fill out the rest of the fields as needed.
8. When you're done, select Save and close. 

Once you're done, create a new invoice for the late fee since it seems that you've already sent one to the customer. Here's how:

1. Go to the + New button.
2. Select Invoice.
3. Fill out the invoice.
4. In the Product/Service drop-down, select the late fee item you created.
5. In the Message on invoice section, include a note about the invoice the fee is related to. Also include the invoice number.
6. Select Save and send or Save and close. 

Also, it's a good idea to tell customers about surcharges and service and late fees for overdue invoices before you invoice them.  You may want to include this in the default email message you send along with invoices. To learn more about this, you can refer to this article: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Please keep me posted if you have other concerns about applying charges to overdue invoices in QBO. I'm always around to help. Take care and stay safe, @userzunaira.shabbir9.