How to Apply UPS Simple Rate?

Everything about this library has been great so far and we haven't had much issues with shipments, but I can't seem to find out how to get the flat rate shipping to work here. I would like to use UPS's flat rate shipping aka UPS Simple Rate, but I can't seem to find where I can do this. I've looked at Entity/Service.php and the files regarding rates, but I can't seem to find any mention of this anywhere. Am I looking in the wrong places? If so, can someone point me in the right direction on how to get flat rates to a shipment? Thanks.