ive never charged for bidding, but we rarely lose any. Actually, i did one time, charge. I was a gut feeling. The offer of course was to credit back the design fee if we got the job. We did get it. So, I say just use your instinct, if it doesnt feel right, explain the design process, the time it takes, the education it takes, the experience it takes and the COST it takes to put together the design. Offer refund on install if you chose that route. If they take issue with it, its not likely a customer you want anyway.