Add it to your cart. When you proceed to checkout, you get a page with various delivery and payment options. Under step 3 (payment) select "edit", there you can choose "invoice", unless for some reason it's not available to you. If I'm not confusing it with another shop, i think invoice was unavailable to me for several years, some time ago, and it's not like I'm one that's late with payments or so. If you can't choose invoice and are not in a hurry, you can pick "advance payment", it's basically the same, apart from the fact they'll ship the article when they receive the payment. It takes one or two days more, just the time for the transaction.