The invoice will be sent as soon as the online shop ships your order, so it's probably just a matter of time. Fortunately, you have chosen Zinia, and you now have time to wait and pay once you have received your order. 

Is the payment date approaching and you still haven't received the order (or haven't even received a notification about the delivery yet)? Please contact the online shop or store where you placed the order. They will let you know the status of your order and when you can expect it.