So, here is the thing:
1. I want to continue to use USPS, BUT I want to use it for USA/domestic orders ONLY.
2. I don't want to offer USPS international shipping any longer, and I want international customers to ONLY have the option to choose FedEx international or UPS international.
Is there a way to do this?
To do that you can create two zones; domestic & rest of the world. For domestic, you can add rate from USPS (see the image below) and for the rest of the world, you can add UPS and FedEx international services only (you can choose the services you require from the list).
I hope you would have enabled carrier-calculated rates on your store, that helps you display shipping rates at checkout. If not, you can choose any of the following options and contact Shopify support to enable this.