Carrier of choice is greyed out, cannot select my preferred carrier.

This has probably been asked before but i cant find the answer.

On some items when i sell them it makes me use a carrier that is not in my selection.

I sell on amazon handmade and have my carrier preferences set to USPS but when i go to fulfill orders it’ll default some of them to UPS ground instead of first class, it’ll let me change it back to First-Class no sweat and other times itll grey out all options aside from UPS not letting me change it to first class or any USPS option at all.

Why does it do this when i have my carrier options in my shipping template selected for USPS ONLY.

Fedex and UPS are a 35 minute drive away, DHL is 45 minutes away if i wanted to sell something international, i have a post office 5 minutes away so i will always prefer to use that, even if it cost me the extra 68 cents.