How do I apply a $400 payment to a customer account when the two invoices are different amounts, neither is $400?

Hi there, 

Since this is your first post in the Community, I'd like to give you a warm welcome!

Don't worry, I'll show you how to apply that payment to the invoice. From your menu screen, click the Plus icon (+) > Receive Payment > Enter the customer's name and email > any open invoices will appear at the bottom part labelled Outstanding Transactions. I'll attach a screenshot for reference below. You can enter the Amount received and delegate the payments appropriately to the invoices you see on the screen. 

Try this out and feel free to shoot me a message if you need help. I got your back. :)