Multiple carrier support is possible and will work using an EasyPost account with multiple carriers. You should enable the Automatically choose the cheapest rate option for this to work (see screenshot below).
This setup will be using a single return address which you can set in the General settings.
You can have multiple custom return addresses through our Advanced Rules feature. You can easily set up a rule for e.g., international orders, which will be redirected to another custom address.
Connecting multiple carrier accounts is particularly interesting for merchants with international stores.