The offsites do it too (Explorers, Holiday Inn, Kyriad and Dreamcastle) and definitely ask for this service
before they ring your purchases up. If you don't they have to refund everything then process it again and although the CM may not mind it doesn't go down very well with the other people waiting in the queue behind you!
If you miss the cut-off time then your goods will be delivered next day which is fine so long as it's not your last day.