Well generally there is no need for you to actually send an invoice. When someone buys something they are presented with a payment screen. If they pay at that time then no invoice is needed as it is paid sale and an invoice, which is essentially a request for payment, would be superfluous. If somehow the order slips through without being paid then you will see the sale on your "sold" page with a link to the left that says "send invoice". Click that and follow the prompts.
If on the other hand the buyer wants something to prove payment (a bill of sale) they can print that right from their own "bought" page.
Hope this answers your question.
"Laissez-faire capitalism (AKA The Great Material Continuum) is the only social system based on the recognition of individual rights and, therefore, the only system that bans force from social relationships." ~ Ayn Rand
