I just got a payment from my customer, there are multiple open invoices, which invoice should I apply my payment to? 

In general, when you receive a payment, you should apply it to the most recent invoice. If the payment amount is equal to or greater than the invoice balance, the Invoice will be set to “Paid” automatically.

If this payment also covers any past invoices, you can directly mark them as paid. Don’t apply a duplicate payment to the past invoices as that will be counted twice in the customer’s billing history.

At any point, if you are not certain about a particular invoice, you can visit the customer’s billing history page to get a complete view of their billing history.