If the vendor would reasonably think that your receptionist had the authority to accept on your behalf, then they could rely on her "apparent authority"; that is, if a person would seem to have authority to reasonable outside third parties, that appearance of authority is enough for them in most cases. You can, of course, dispute that she ever in fact authorized this; then it will come down, if they sue you, to how is more credible and/or has supporting evidence (e.g. a recorded call)--the vendor or your receptionist.If she did in fact authorize this while knowing that she did not have authority, you could sue her to recover the cost from her--she acted outside her authority, and so could be liable.