The cutoff is the order, not the line
The 3pm cutoff is the same for a single carton and for a mixed order of boxes, tape and void fill. The order leaves together when it is placed by 3pm. It is not split so that the tape waits for another day.
UK delivery is free when the goods total is over £50 ex VAT. Mixing the lines on one order is how that total is reached. The threshold is the goods, before VAT.
What each line is for
The carton is the outer wall. Void fill stops the goods moving. Bubble wrap or foam covers a fragile face. Tape closes the pack. Those are different categories, and they can be one basket.
A large or awkward consignment, such as a pallet of cartons, is described on the delivery page. Read that before assuming a mixed pallet follows the parcel notes only.