I have issue with USPS First Class Package Service calculated rates.
USPS First Class Package Support up to 13 oz packages. However i don't know the dimension limits.
Im trying to calculate shipping cost for 1x1x1 inch 10 oz package. But it says
"There are no shipping methods available for your cart or destination."
However, Ive done some testing and found out that when the package weight is under 4oz it works. But the shipping cost is still much higher - $4.66. (Shopify USPS First class rate is $2.88 for 4 oz) https://help.shopify.com/en/manual/shipping/shopify-shipping/rates#first-class
Please notice, if I turns on USPS Priority Mail there is no issue.
First-Class Mail envelopes and parcels max weight is 13 oz, and while I can't find restrictions that outline the package size, I don't suspect that your 1x1x1 will cause much of an issue (dimension wise). That said, the weight of your package (10oz) is the most likely factor in the rates not appearing at checkout.
When the checkout sends a signal to USPS to retrieve the rates available for that specific cart it will take your product weight and package weight into account. Taking a look at the products you offer, it does look like these products may weigh more than 3oz which would put you past weight limit.
Have you considered reducing the weight of your package to say 9.5, or even 9.99 to see if this changes the results at all? The system may be a bit sensitive to being right on the 13oz line, so altering the weight by a small fraction may prove better results.
Let me know how that turns out!
I checked with our developer (we have a shipping calculation app) and he said that if what is being passed to USPS is under 13 oz. (12.99 oz.), it should be returning rates.
As Peter noted, the weight of your envelope, as well as your product packaging will contribute to the total oz. being passed to USPS, so this would likely be the culprit of what is causing the issue.
I recommend contacting Shopify’s chat support or phone support to get this resolved since it sounds like an issue with your setup. They can go over your product settings and shipping settings with you to try and pinpoint what the issue could be.
I can do some testing on my side here to see If I can spot the issue, however, to do this will require that we authenticate the account. I'll ship you off an email here shortly requesting approval to your store; if you could follow up there with your authorization then I can start digging into this issue further!
I have a slight disagreement regarding the statement that shipping weight must be under 4 oz to make USPS first class rates to work. If you are using a box, then just make sure together with the order weight the total package weight does not exceed 13 oz. For example, i just tested this by providing a weight of 10 oz inside USPS price calculator and got the First class rates perfectly as shown from the screenshots :
In addition, to this, i have even verified these services using the Multi Carrier Shipping label app and got the rates.