Customer Statements

Hi im trying to figure to select invoices for a statement, eg. customer has 15 invoices come mid month and i only want to select those 15 to charge now (without selecting previous months invoices), and then the remainder of the month how ever many invoices we do on a seperate statement?
our customer would like a statement every 15 days with a 30 day payment period so im trying to work with that. Thank you