[dakiol]

I think this suggestions are fine but none of them solve the problem here. The "client" (the service the OP owns) can be as atomic and transactional as one wants, but if the "server" (the 3rd party service being called by the "client") doesn't offer either a) idempotency or b) a retrieval mechanism for existing resources, then the "client" can't do anything about the original stated problem.