Regarding the references provided by Taña:<p>
https://obedalvarado.pw/blog/formato-de-factura-simple-con-e...This describes a software template, not necessarily equatable with paper format. In this case it is translatable to a simple invoice (as in not a complicated design, or not easy to fill-in and/or use), particularly as backed-up by the follow-through link
http://obedalvarado.pw/simple-invoice/ .<p>However, I don't think this is a good source for backing up the term for the paper format product in the OP; usage is different. Paper format has single-sheet, duplicate, triplicate etc. that doesn't exist as such for SW invoices (again, it's a different format).<p>
https://www.proz.com/kudoz/english_to_spanish/law_patents/11...There's nothing in this link in reference to whether it's single-sheet or simple, just that it's a book. I don't think that's in discussion?