Quick seller check
Can this go as a large letter?
Measure the final packed item, then compare every side with the 353 × 250 × 25mm limit and the 750g weight limit.
Thickness is usually the trap
If it is over 25mm or over 750g, it does not fit the published large-letter limits.
Soft items can expand after packing. Do not force an item through a slot; use the next format if it is tight or the carrier says it fails.
Royal Mail large-letter limits ↗
Check exact measurements → · Large letter size · Small parcel size