We have found a possible bug with the USPS carrier-calculated shipping rates method.
We have created a development store at https://shipping-rates-issue.myshopify.com/ with only one product, a brick that weighs 1 lb:
Only the USPS carrier-calculated shipping method is enabled.
For testing purposes, the shop address is 350 5th Avenue, New York, NY, 10118, United States and the delivery destination is 150 Elgin Street, 8th Floor Ottawa, ON, K2P 1L4, Canada.
Everything works OK if less than 66 bricks are added to the shopping cart. If you add 66 or more, the USPS shipping rate is not available:
Surprisingly, if you configure the brick to weigh 10lb, the issue is not exhibited. You can add 10,000 ten-pound bricks but you cannot add 100 one-pound bricks.
Could you clarify if this is a bug? We have a customer with those shipping rules that is experiencing the issue.
On the other hand, if more shipping rates methods are configured (say USPS, UPS and FedEx), the checkout simply does not show USPS and shows the other shipping options (UPS and FedEx). However, the Ajax API /cart/shipping_rates.json method simply gives a generic error, not saying that UPS and FedEx are still available.
Code Black Belt
That definitely sounds like an issue. I know Priority Mail International has a weight limit of 70lbs, so in both cases you brought up there is unexpected behaviour. I can only see a generic "no rates for your destination error" when I look behind the scenes, which isn't very helpful. Can you have your client create a ticket with our support team? You summed up the issue nicely here, so it might be worthwhile for them to include it in their email/call/chat.
I am pretty sure you will not face this problem again. That means 66 or 67 Bricks can be easily added in cart and you should get the rates without any problems. However, beyond 70 lbs there may be a problem as then Shopify would have to double the shipping cost as 70 lbs is the weight limit of USPS Priority box. Beyond that a new box would have to be used. If you are looking to pack items in Small, Medium and large boxes based on the dimensions/weight of the shipment, then you need to use the Multi Carrier Shipping label app as with the app you items are packed based on dimensions. So if a particular item fits inside Small flat rate box, this will be used. Also the algorithm works well for the second packaging as well. For example, if you 3 items have gone inside medium flat rate box and only 1 is left which can be packed in a small box, then the app will select a small box for this purpose.
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