Matt,
WHMCS gives the following:
RESULT => 0
PNREF => xxxxxxxxxxxx
RESPMSG => Approved: 10751-There's an error with this transaction. Please enter a valid state in the billing address.
AUTHCODE => 111111
AVSADDR => Y
AVSZIP => Y
CVV2MATCH => X
PPREF => xxxxxxxxxxxxxxxx
CORRELATIONID => xxxxxxxxxxxxxx
IAVS => N
The invoice is not marked as PAID by WHMCS, and therefore WHMCS will attempt to bill it next time and again, and again etc. Changing all our client states to the two letter abbreiviation fixed it.
If you could check this over it would be appreciated.