If you want to use Priority flat rate, you have to use Package preferences and select the flat rate options.
You also have to have a shipping profile for Priority for this to work.
You can put any method you want in a profile.
Only FC or FC and Priority or these 2 plus express.
For items over 1 lb, you can only have Priority, express, parcel select..
FC wouldn't work if it's over 1 lb.
I have 1 profile for FC and Priority for items under 16 oz.
That way a buyer can pick between these 2 methods.
PLUS if a buyer adds to their cart enough FC items to be 16 oz or more, they no longer see that FC option, they'll only see Priority.
Then I have another profile for items 1 lb or more and only have Priority.
Here is some info on one of the other threads:
+ How do I add Flat Rate or Regional Rate options? +
If you’d like to use Flat Rate or Regional Rate options for shipping your items, just add the Flat Rate or Regional rate package type that you will use in the Package Preferences section on the Shipping Profiles Manager page. Make sure that you offer mail classes that support Flat Rates and or Regional Rates like Priority Mail, Priority Mail Express, and their international equivalents.
When a buyer selects the associated mail class, the Flat Rate or Regional rate cost will automatically be calculated for him or her, as long as the order is within the Flat Rate weight and dimension limits.
Learn more in the Package Preferences FAQ here: www.etsy.com/help/article/6167
Here's a link to the entire thread with a lot of helpful info.
https://www.etsy.com/teams/23557/calculated-shipping-prototype/discuss/16154340/page/1
