A) We had pet insurance but still owed $40 in fees. B) We had pet insurance: but still owed $40 in fees.
C) We had pet insurance but, still owed $40 in fees.
D) We had pet-insurance but still owed $40 in fees.

I think it's be your welcome

A. is correct.

Explanation:

Don't be confused (or fooled) into thinking that you need a comma before the coordinating conjunction 'but.'  You would only add a comma if that second clause contained a subject, e.g.

We had pet insurance, but we still owed $40 in fees.

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