Thanks for reporting that problem with our beta Invoice feature; we've reproduced it and will report that to our Devs. [Edit: turns out it was caused by the affected item having insufficient stock available.)
To clarify, that feature is meant for sellers to manually build an order which will be paid later online or offline (e.g. by check or purchase order), or could be sent to the buyer as a link they could use to review the order and then pay for it online.
If you just needed to provide an invoice for a buyer who's already placed their order online, that would be the Common Thank-you Page we generate automatically for every order we process. You can obtain a link to that page by finding their order in your E-junkie Transaction Log, click the Copy Link button for that order, then paste that link into a personal message to the buyer. The buyer also should have received a Thank-you Email from us with a link to that page, which is also the page where we provide any download link(s) they purchased.
To build an order to send to a buyer for payment, you can just use your E-junkie Add to Cart buttons to build that order yourself, then click the Share icon near the top-right corner of the cart (sideways-V icon next to the cart icon) to copy a link the buyer can use to replicate an identical cart order, then they'd just take that cart to checkout online like any other.
Or for a single-item order, you don't even need to do that; just go to Manage Products > Get Purchase Buttons/Links in your E-junkie Dashboard, select the product and the type of button you want, copy the Text Link we provide there, then paste that into a message to the buyer.