If you've connected multiple accounts for the same carrier, you've likely noticed the "(Primary)" indicator tacked on to one of your accounts. By default, ShipStation uses a Primary account for everything (like getting rates and shipping orders) unless another account is specified.
If you want to ship an order with a carrier account other than the Primary one, head to the "Other Shipping Options" section of the Order Details screen and you can select a different account from the Shipping Account dropdown.
If you don't see the "Shipping Account" dropdown menu on the Order Details screen, select a shipping service, then double-check that you have multiple accounts connected for the carrier's service.
While you can switch between your different accounts for a carrier while shipping an order, you cannot switch between your accounts everywhere in ShipStation. There are areas that are only available for the Primary Account. These areas include (but aren't limited to):
Are you shipping with another account more than your primary? It's easy to make another account the Primary account.