When a bag is not enough
Soft goods can leave in a mailing bag. Goods that need a clean rigid outer can take a white postal box: 125WK/T, including shell-and-slide packs with foam. The category says those white postal boxes qualify as Royal Mail small parcels.
The outside maximum is still 450 x 350 x 160 mm. White board does not change it.
On the same order as the fill
The box, the void fill and the tape can be one Packaging Suppliers order. Placed by 3pm, it is dispatched that day.