In principle, items should be rectangular and the longer side should be at least 1.35 times and at the most 2.6 times as long as the short side.
Square items with a side length of between 140 mm and 176 mm can be processed mechanically.
Non-rectangular or square items are regarded as special items.
Empty sections of covers get crushed, potentially resulting in damage to the contents or malfunctioning of the processing equipment.
Avoid using unnecessarily large envelopes.