USA – Selling into USA via Amazon…. Any reason I can’t offer Free Shipping to anyone that buys my product in USA?

I’m not sure if this is a dumb question or not. I’ve just been doing some calculations and think I can sell a product for about 15.99 or 17.99…. I have found some shipping through the USPS that appears it would be max about $5.33 on 9-12 oz 1st class package to “ZONE 9.” I would be doing fulfilled by merchant starting out…

Is Amazon even going to let me sell only to USA customers on their platform? I can’t have someone buying my product and trying to ship it to Canada or mexico or Australia or anything like that… I basically would initially want to limit it only to the USA and its 50 states that USPS would deliver too….

Any tips for me? Thanks