Short version: SureCart is sending your quantity to Shippo, and it re-quotes on every change. The price stops moving because the whole cart is quoted as one parcel using your default box, and USPS prices that box in wide bands. Inside a band, extra pounds cost nothing. Everything below is a live SureCart store with a real Shippo connection, quoting the exact address from your screenshot.
1. Proof that quantity is reaching Shippo
I drove a real checkout and read back what SureCart calculated at each quantity. Product = 1 lb per unit. Default box = 12 x 10 x 6 in, 0.5 lb empty. Destination = 12 Autumn Ridge Ct, Katonah, NY 10536.
| Quantity | Weight SureCart sent | USPS Ground Advantage | Fresh quote? |
|---|---|---|---|
| 1 | 1.5 lb | $8.17 | yes |
| 2 | 2.5 lb | $8.17 | yes |
| 3 | 3.5 lb | $8.96 | yes |
| 4 | 4.5 lb | $10.01 | yes |
| 5 | 5.5 lb | $13.98 | yes |
| 6 | 6.5 lb | $13.98 | yes |
Notice rows 1 and 2. Same price, two different weights, two separate live quotes. That on its own looks exactly like the bug you reported, and nothing is broken.
2. Try it yourself
This is a live checkout on this store, in test mode, wired to a real Shippo account. Open it, enter any United States address, then use the + button in the order summary to raise the quantity and watch the Shipping line.
The cart is remembered between visits. If you see something else in the summary, click Remove first.
3. What actually freezes the price: the box
I ran the identical product, the identical quantities and the identical address four times. The only thing I changed between runs was the default parcel template.
| Default box | qty 1 | qty 2 | qty 3 | qty 4 | qty 5 |
|---|---|---|---|---|---|
| 9 x 6 x 3 in | $8.17 | $8.17 | $8.96 | $9.02 | $9.02 |
| 12 x 10 x 6 in | $8.17 | $8.17 | $8.96 | $10.01 | $13.98 |
| 14 x 12 x 8 in | $8.17 | $8.17 | $8.96 | $10.01 | $15.16 |
| 18 x 14 x 12 in | $44.33 | $44.33 | $44.33 | $44.33 | $44.33 |
Two things are going on, and both come from the carrier rather than from SureCart:
- Dimensional weight. A big, light parcel is billed on its size, not its contents. Until the real weight overtakes the size, adding units changes nothing.
- Rate bands. USPS does not price per ounce. It prices in steps. Two weights inside the same step cost the same.
On the 12 x 10 x 6 box the price sat at $13.98 the whole way from 5.5 lb to 16.5 lb, then jumped to $37.96 at 20.5 lb and $72.61 at 25.5 lb. The jumps are real. They are just further apart than you would expect.
4. The one thing to check on your store
SureCart > Settings > Shipping > Warehouses, and look at the parcel template flagged as Default. That single box is used for every quote, for every order, no matter what is in the cart. If it is bigger than what one or two bags of coffee actually ship in, it is dictating the price and the weight is irrelevant.
Set it to the box you really use for a small order. The rate will start moving sooner. It still will not move on every single unit, because of the rate bands above.
5. How to see the weight SureCart is sending
If you want to confirm the weight yourself rather than take my word for it, add a temporary weight based rate to the same zone with narrow bands, for example 0 to 2 lb, 2 to 4 lb, 4 to 6 lb, each at an obviously different price. It reads the exact same cart weight the Shippo quote uses. Raise the quantity and watch which band it lands in. Delete it afterwards.
6. The other problem: one address falling back to the flat rate
Katonah NY 10536 quotes perfectly here, so the address itself is fine. Worth knowing: the admin shipment screen and the checkout do not ask the same question. Admin quotes your warehouse straight to the destination and shows you everything that comes back. Checkout first has to find a shipping zone that contains the address, then it keeps only the carrier services you ticked on the live rate. Either filter can empty the list, and when it does the live option simply disappears and whatever flat rate you have left is what the customer sees.
I reproduced both on this store:
- Zone does not cover the address. I narrowed the zone to New Mexico and quoted the New York address. Shipping choices returned: zero. Check Shipping > your profile > the zone holding the live rate and confirm New York is inside it. A zone set up per state, or with a postal code list, will quietly exclude everywhere else.
- Selected services do not match what the carrier returns. I left the live rate pointing at a service USPS does not offer domestically. Shipping choices returned: zero, with no error anywhere. Open the live rate and make sure USPS Ground Advantage is ticked, since that is the service your admin screen quoted at $9.54.
One practical note: after changing a zone or a rate, start a fresh cart to test. An open checkout holds on to its last shipping quote for a while and will not pick up the change.
