A courier is a company that moves a specific item between two named points on a service with a stated time and a tracking record, which is a narrower thing than the post. The post accepts anything addressed and moves it through a network optimised for volume; a courier accepts a booking, collects, and is answerable for that consignment. The distinction is not academic, because it is what the price difference between the products in this record is made of.
What separates a courier from the post
Booking, collection and accountability. A courier's service starts with a booking that names the item, the two addresses and a time, and most of them collect rather than waiting for you to visit. The post starts when you hand something over. That is why Royal Mail's grid begins at 91p for a 2nd Class Letter and a courier's cheapest published parcel in this record is £2.39, and why the two are not really competing at that end.
Private delivery services, and what a document courier service is
A private delivery service simply means a carrier other than the state postal operator, which in the UK is most of the sixteen companies here. A document courier is a narrower product for time-critical paper: contracts, passports, legal filings, where the value is in the chain of custody rather than the weight. It is priced as a service rather than by band, which is why nobody in this record publishes a figure for it.
Multiple parcel delivery
Sending several parcels at once is normally cheaper per parcel than sending them separately, because the collection is shared, but it is not automatically a discount: on the published bands each parcel is still priced in its own band. Where several boxes go to one address, the question worth asking is whether they should be a pallet instead, which is priced per pallet rather than per box.
Other questions people ask
What is a courier?
A company that moves a specific item between two named points on a booked service with a stated time and a tracking record, usually collecting rather than waiting for you to bring it in.
What is the difference between a courier and the post?
Booking, collection and accountability. Royal Mail's published grid starts at 91p for a 2nd Class Letter; the cheapest published courier parcel in this record is £2.39, and the two are not competing at that end.
What is a private delivery service?
Any carrier other than the state postal operator. In the UK that describes most of the sixteen companies in this record.
Is multiple parcel delivery cheaper?
Per parcel usually, because collection is shared, but each parcel is still priced in its own published band. Above a few heavy boxes to one address, ask for a pallet quote instead.