What shipping carrier gives the lowest rates for shipping parcels under 1lb in the USA?

Out of all of the third-party shipping carriers, USPS First Class provides the cheapest rates hands down.

FedEx and UPS can't compete with USPS on packages under 1 lb. The Post Office was founded on sending small parcels and letters, so they have no problem making a profit. The other companies’ infrastructures don’t enable it. No one will beat their price unless you send more than $10k/month. With DHL eCommerce, Globalmail, etc. you can save on parcels between 1.5 to 3lb, but I think you have to commit to sending at least 500 parcels/month. Also, some people complain about losing packages with them, so take it into account. With EasyPost (they have a Shopify app), you can compare rates between more than 100 carriers and bring your price down. Check it out. Another option you should consider is to use a fulfillment center. They manage inventory and shipping for you, and their rates are really low. Amazon FBA is an option you can look at.