Was it an invoice? Pay them with coins
C fold, Z fold.. Envelope size.
On the other hand, due to their cleverness, you did notice the statement/invoice, didn't you! In a work from home environment, maybe the business owners 6 year old was tasked with folding, inserting and affixing postage!
Was the statement was set up Landscape? not portrait, so perhaps the folder didn't want you to open up a "sideways" statement?