Hi,
As I'm not familiar with inches (never used and don't understand what is 5-3/8, etc), I've converted everything to centimeters. Sorry for that but I want to be accurate and not make mistakes.
On this page (
store.usps.com/store/product/shipping-su...rate-box-P_SMALL_FRB
), I find that the Small Flat Rate Box size is: 5-3/8 in x 8-5/8 in x 1-5/8 in
This corresponds to a size of : 13.6 cm * 22 cm * 4.1 cm
On your test website, the product Agrotera AGT-1A has a size of: 5 * 3 * 1.25 in.
This corresponds to a size of: 12.7cm * 7.6cm * 3.2cm
To calculate the limitations on the size, we use the length + girth calculation from USPS: ( width + height ) * 2 + length.
The Small Flat Rate Box has a length girth of (13.6+4.1) * 2 + 22 = 57.4
Your product has a length girth of (7.6+3.2) * 2 + 12.7 = 34.3
As 34.3 * 2 > 57.4, once you add two items of that product in the order, it goes over the length girth of the package and thus it requires a second Small Flat Rate Box.
Now, the fact that you can pack more items in the Small Flat Rate Box than what is calculated sounds strange to you but packing is a complex matter:
en.wikipedia.org/wiki/Bin_packing_problem
The solution we use is a simple, crude, one which roughly works but could definitely be optimized to reduce the calculated number of packages necessary for the cost estimations.
I actually looked further into it and I think it could be interesting to implement such library to calculate the packing in the future:
github.com/dvdoug/BoxPacker
In the short term however, that's not something you or we could fix by changing a few lines of code in the USPS plugin or in HikaShop's shipping system.