I have only now noticed that I read 'brought' for 'bought', but the rest of my question stands.
The system marked your answer "habríamos traido" wrong because, as you said, you're using the wrong verb (traer, instead of comprar), otherwise it would have marked it correct.
I checked all test-questions for that lesson and they are all giving the option of the subjunctive and the conditional. Bear in mind though that that flexibility is only possible in the main clause, not the "si" clause. The si clauses only admit the subjunctive. It could be that your other answers were to do with the verb for the si clause and that would have been marked wrong if you used the conditional.
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