Where is my carrier coming from

In Readysell cards

Readysell card addresses allow you to have a carrier per address not just per card.  

It is not based on Web ID.

There are Carrier fields per address on the Addresses tab

There is a Carrier field on Settings, tab Customer Settings - this is for the every address that is flagged as delivery unless the address has a specified carrier on it.

From a web point of view of how it is all set up

When the customer placed an order, the customer may select one of many BILLING and one of many SHIPPING addresses
The 2 addresses that may be the same or different are both passed to Readysell via the XML file
I understand that Readysell may or may not use the 2 addresses provided in the XML file - depending on the store setup
The Shipping Method is set to COURIER 

When the web order comes into the Readysell Sales Orders module

When the web order hits the Readysell sales module, the carrier from the web address goes in as a note against the salweb order.
The carrier used on the Readysell salweb side of things, i.e. visual when you go process the shipment to invoice, uses the carrier on the Readysell customer card address that matches the address that came in on the web order.