USPS firt class shipping can only be selected from your previous items.

[siriussales]

I am sure most have figured this out, but just in case. I started listing items and USPS first class shipping (my go to) was not available.  As a seasonal seller only, I had to select Standard shipping USPS ground advantage as the cheapest option which is significantly higher than first class for my smalls (the base of my items).  I relisted unsold items and found the first class shipping option was in them.  I have been able to use first class shipping by selecting "sell similar items" when I list something new.  Pain in the butt initially but once you have your  "new"similar items to relist, the first class  option goes into the new listing the same as before. Merry Christmas and happy new year folks.  😄 

[winklers549]

Usps First class shipping was moved to ground Advantage a while back. Just update you shipping policy to USPS Ground Advantage

[evry1nositswindy]

The price for Ground Advantage is the same as First Class.  Only the name changed.  You can also use it for heavier items, as it also replaced Parcel Select.

[nobody*s_perfect]

In case the other posts weren't clear:  USPS no longer offers USPS First Class package shipping, which is why it doesn't appear on the listing form any more.  FC Package shipping was merged into Parcel Select and the new combined service is called "Ground Advantage."  Prices are the same as they were for First Class packages. 

This merger was implemented last July.  USPS and eBay are giving users plenty of time to update their listings but eventually your relists of First Class package shipping will probably show "Shipping service not specified."