I will be needing to routinely fulfill orders to customers by shipping.  Having never done this before (literally - never done ANY commercial shipping to anyone), I have a question I hope someone can just give me a little insight as to what my options are.

I will be  using poly mailers (like those little dirty bags you get Amazon and Kohl's orders in) for VERY lightweight apparel (i.e., 3 cloth face masks will be the max contents of package--close to weightless!)

I will be shipping one bag at a time, here and there, domestically within US. 

What are my shipping options?  What is the cheapest way to ship them?  I'm so confused when I go to USPS.COM and look at the various options.  Could I get away with calling this "flat"?  It's kind of pretty close to being flat...

I'm thinking the cost of shipping these should be under a $1, but is it?  What are my options?