What goes wrong
Sending a heavy item in single wall because it was the size you had is how the board gets the blame.
Packaging Boxes lists Single wall boxes for this line. Double wall is the stronger board, manufactured in the UK, for heavy, valuable or fragile goods and for a stack. A bigger single wall box is not that board.
The check before you reorder
Single wall is the everyday board: strong, affordable and versatile, for a normal dispatch weight.
Store sizes in columns. The print on the bundle is the size. The end of the bundle is not.
Dispatch on that order
A Friday order on Packaging Boxes that includes Single wall boxes aims for Monday. A bank holiday on Friday or Monday moves that delivery. The 3pm cutoff is still the same-day dispatch rule.