Typically, the couriers can arrive anytime between 9am and 5pm; or at the very latest sometimes 6pm.
If you place the shipment to be collected before 2pm then they should come between 2pm and 5pm the same day but if you send it slightly after then it may roll over to the following day.
If your cut off time is 3pm and you book the shipment just outside that time period then the driver may come the following day. However, if this is done on a Friday then as the couriers do not do week-end pick-ups then it roll-over to be collected on Monday.
It is also worth noting that if you have a cut off time of 3pm and you book the shipment at 2pm there isn't a guarantee that they may come on the same day to collect this.
What you could also do is exchange contact with the driver; so you can call to work out the best time to collect your parcels; otherwise it will be added to the route and the driver can come at anytime.