Don't send another invoice or contact them again.
After 5 days, you can cancel using the reason 'buyer did not pay'.
As long as you wait and cancel for that reason (it won't appear on the dropdown menu as a reason until day 5), they will get an unpaid item strike and won't be able to leave feedback.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)