When using flat rate boxes, which items are excluded from the standard size?

The correct response highlights that items which are liquid or perishable cannot be shipped using flat rate boxes. This is primarily due to safety and handling concerns. Items that are liquid may leak or spill during transit, potentially damaging other packages or creating hazardous conditions. Perishable goods, on the other hand, require special handling to ensure they remain fresh and safe for consumption, making them unsuitable for the flat rate service that does not offer temperature-controlled transit.

In contrast, other choices present items that are typically allowed under flat rate shipping. For instance, while bulk items weighing over 50 lbs would be subject to additional charges or different shipping alternatives, they do not outright exclude the use of flat rate boxes in terms of size. Similarly, envelopes and postcards, as well as documents without physical items, correspond to different mailing services rather than the exclusion criteria for flat rate boxes. Flat rate shipping encompasses a broad range of items, provided they are non-perishable and comply with weight limits and size constraints.