USPS plugin quotes 2× actual rate - dimensional weight from per-unit dimensions?

Hello,

1.
Regarding the packing logic, where the 2x to 3.9x discrepancy almost certainly comes from, HikaShop does not have a "shipping box catalogue" concept. It does not know about your Small (1,440 cu in) or Medium (3,584 cu in) USPS boxes. What it does instead, for every quote request, is:
- Take each product's per-unit dimensions (length, width, height) and weight.
- For an order line of N units of one product, build a virtual package by stacking N units along the product's smallest dimension. So for example 6 units of a 4 x 6 x 10 in product become a single 24 x 6 x 10 in virtual package. The other two dimensions stay at the single-unit max.
- Split that into additional packages only when a hard USPS limit is reached (provided by the plugin): 70 lb total weight, or 130 in length + girth.

2.
HikaShop doesn't compute DIM weight on its side. It sends raw weight + length/width/height to USPS, and USPS applies its own DIM weight rule on their end.

3.
There is no built-in way to map "Product X => Small Box" or "Product Y => Medium Box". The dimensions sent to USPS come from the product record, not from a box library. The two workarounds we suggest to merchants in your situation are:
- Set each product's dimensions to the dimensions of the shipping box that will actually be used for one unit of that product (so a soap that goes in a 6 x 6 x 4 mailer should be configured as 6 x 6 x 4, not as the bar itself). HikaShop's stacking logic for multi-unit orders then approximates a stack of those boxes, which is usually closer to the truth than stacking the bare products.
- Use a "Group packages" toggle on the plugin (setting "Group packages") and the per-API "Packaging weight" settings (Letters / Flats / Packages) to add the empty-box weight on top, so the rated weight matches what you actually put on the scale.
The "Group packages" option is enabled by default; when disabled, each unit is rated as its own package, which would absolutely explain a 2x+ overquote on multi-unit orders. Worth a quick check that it is on for your configuration.

A few other usual suspects for 2x+ over-quotes on Ground Advantage:

- "Price type" in the plugin settings: should be Commercial in your case (you mentioned you use Click-N-Ship Commercial Plus). Retail would be substantially higher.
- "Weight approximation" and "Dimension approximation" inputs in the plugin: any non-zero value here is added on top of every package's weight / smallest-dim and can quietly inflate the rated package.
- "Has nonstandard characteristics" toggle: if on, USPS adds a nonstandard surcharge that you probably do not pay on your own labels.

To investigate your specific 2x to 3.9x gap, what would help us most is one concrete order from your data:
- The list of products in the cart, each with their HikaShop product dimensions (L x W x H, units) and weight (units) as stored in the backend, and the quantity ordered.
- The shipping destination ZIP code.
- The rate HikaShop quoted at checkout, and the rate Click-N-Ship actually charged you on the label.
- A screenshot of the USPS plugin's settings page, especially the Price type, Services, Group packages, the approximation fields, and the packaging weight fields.

4.
Yes, the plugin we released last year at www.hikashop.com/marketplace/product/290-usps.html uses the new API.