Glad to see you here in the Community, @thanhphuongy. I'm here to lend a hand with your concern about invoices in QuickBooks Online.
For your first question, you can only pay one invoice for e-invoicing. Otherwise, the customer can edit the amount next to the exact amount in the e-invoice. That said, they can pay all their outstanding invoices all at once.
See the sample screenshot below:
To answer your second question, you can let the customer view their updated invoice outstanding balance below the Payment amount.
Please see this sample screenshot for reference:
For further details about processing payments, feel free to check out this article.
You may consider reviewing this article about personalizing your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Keep me posted if you still have questions or concerns about your invoices in QBO. I'll be around for you. Have a great day!