Clients get very confused with those terms and use them incorrectly all the time. Multiple sounds like when they have a multiple page document so they use that to do a 3 page invoice and not "combine". I think that you should get rid of the multiple and just have single and combine. If they have a bunch of unrelated invoices they should do them separately anyway. On email when they have a bank statement that is many pages they need to send it to the "single" email which is the opposite of the phone app. The description to email a multiple page doc is worded as if it is for a document that has multiple pages and not one that needs to be split. All around confusing.