The only reason for sending an invoice is to change or combine postage, if you are trying to do this, there is a technical issue some are experiencing, and this message from a Community Manager here:
We are aware of the issue and its currently being looked at for a fix. We don't have an exact timescale right now but its being worked on.
In the meantime you may need to refund the shipping difference to your buyer after the purchase.
@nickc4007
