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Shipping rate calculator

Shopify Shipping Rate Calculator — Real Rates, Real Time

Most Shopify stores use flat-rate or weight-based shipping tables that don't reflect what carriers actually charge. The result is invisible margin loss: you undercharge on heavy or oversized orders and overcharge on small ones — driving up cart abandonment on one end and eroding profit on the other. PackOS replaces guesswork with math. It runs a bin-packing algorithm at checkout to determine the exact box for each order, then queries USPS, UPS, and FedEx with real dimensions and weight. The rate your customer sees is the rate you'll pay for the label.

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Calculate my box

Items
ItemLength (in)Width (in)Height (in)Weight (lb)Qty
Item 1
Boxes
BoxLength (in)Width (in)Height (in)Empty weight (lb), optional

Edit these to match the boxes you stock.

Rates come from your own carriers once PackOS Checkout is installed.

How It Works

  1. 01

    Install PackOS from the Shopify App Store

    Add PackOS to your Shopify store. The app registers as a carrier service so Shopify calls it automatically at checkout.

  2. 02

    Configure your box sizes and carrier preferences

    Enter the boxes you actually stock — their interior dimensions and weight limits. Select which carriers and services to show at checkout.

  3. 03

    Rates appear automatically at checkout — accurate, real-time

    When a customer checks out, PackOS runs bin-packing to find the optimal box, queries carrier APIs with exact dimensions, and returns live rates in real time.

What PackOS Calculates

  • Optimal box size via bin-packing algorithm — selects the smallest box that fits all items from your library
  • USPS rates — Ground Advantage, Priority Mail, Priority Mail Express
  • UPS rates — Ground, 3 Day Select, 2nd Day Air, Next Day Air
  • FedEx rates — Ground, Express Saver, 2Day, Standard Overnight
  • Multi-box shipments — automatically splits orders that exceed a single box
  • Weight-based carrier selection — uses actual weight and dimensional weight, whichever is greater

Frequently Asked Questions

How does PackOS calculate shipping rates at checkout?
PackOS installs as a carrier service on your Shopify store. When a customer reaches checkout, Shopify sends the cart contents to PackOS. PackOS runs a bin-packing algorithm to determine the optimal box from your configured library, then queries USPS, UPS, and FedEx APIs with the exact dimensions and weight. The live rate is returned to checkout in real time.
Do I need a Shopify Advanced plan for carrier-calculated rates?
PackOS works on any Shopify plan that supports third-party carrier-calculated shipping rates. This includes Advanced Shopify, Shopify Plus, and stores using the Carrier Service API. Check your Shopify plan details or contact PackOS support if you are unsure.
Does PackOS handle dimensional weight pricing?
Yes. Because PackOS knows the exact box dimensions for every order, it automatically accounts for dimensional weight pricing from UPS and FedEx. A 2 lb package in an oversized box will be rated by its dimensional weight, not its actual weight — and PackOS calculates this correctly at checkout.
What happens if an order needs more than one box?
PackOS supports multi-box shipments. If the items in a cart do not fit in your largest box, PackOS splits the order across multiple parcels, calculates the rate for each parcel, and returns the combined total to the customer at checkout.

Get PackOS Checkout

14-day free trial on the plan you choose, billed through Shopify.

Get PackOS Checkout