I only use USPS, as my products are light and small. First class envelope does not work as my items usually exceed the height requirement.
So I use First class package as my shipping, and give priority and priority express as alternatives to quicker shipping.
I have all 3 rates chosen, however when I go to checkout, First class package only works on some of my items, not all of them.
I do not have special rates for each product, and its is mere ounces that differ. I cant open my shop because I would never ask someone to pay more for shipping then what I sell my products for. Not this anyways.
I had the same issue and have found a fix.
I only used USPS First Class to ship my products. I use a 9x9x6 box as well.
I found that when you go to the unfulfilled order and create a shipping label, you will see your products and the weight.
My weight wasn't populated correctly and once I input the correct weight, the First Class USPS option appeared.
Having discovered this, I will now put the correct weight per item and the correct weight of my box (already saved) to prevent this issue from happening again.
Hope this helps someone here.
If the height of an item is causing an issue while choosing the package while shipping,
You can solve this by opting for a stack-packing method from the Multi-carrier shipping label app.
The stack-packing method of the app lets you create a custom shipping box based on the height of the product.
After creating a custom box by stack-packing method from within the app within Shopify, the app automatically chooses the cheapest shipping rate for the product with the right custom box and displays real-time shipping rates with different service options from multiple carrier services like FedEx, UPS, and USPS at the checkout.
As the app chooses the cheapest shipping rate, your customers are devoid of paying more shipping charges.