My primary experience with supplier surveys requested by customers was with automotive and aerospace customers.
My company was self-declared compliant to an ISO or QS Standard, so we were not able to slide by ticking off our registration number and registrar. In defense, I did as Jim Wynne did and created a super slick self assessment survey based on the QS9000 Standard.
Each page was divided into 3 columns. Column 1 was the text of the Standard; column 2 had a detailed explanation of how we met the Standard; column 3 was blank. We invited ALL customers and prospects to drop by anytime (we were open 24/7), armed with the copy of the survey and invited them to compare any or all of the clauses with our actual operation for themselves and fill n the blanks with their own observations.
The darn thing worked very well - never had a single prospect or customer take us up on the challenge. After the first five uses of the Self Assessment Survey (SAS) in response to requests, we got such complimentary comments, we decided to have the thing printed up and bound to use as a sales tool. Once we started using it in advance, we never received another request for a SAS from any customer or prospect.
The problem of customers asking for confidential financial or sales information can be a very touchy subject. We were not a publicly traded corporation and considered information about our customers as trade secret (both our own secret and the secrets of our customers.) Our solution was to "inoculate" customers and prospects to prevent them asking the question. I no longer remember the exact language, but the gist of it was,
"We are a privately held corporation and do not disclose our financial information to any entity which has not signed a confidentiality agreement binding all principals, partners, employees, servants, and agents. We recognize some customers ask such questions out of concern whether the supplier has sufficient financial capacity to service projected orders. If your annual order will exceed one million dollars, we will supply a performance bond guaranteeing our performance if you will provide a similar bond guaranteeing your performance in purchasing.
"Some customers ask about who and how much business we do with other companies. We consider such information a trade secret and do not disclose such data. Please do not ask."
This "immunization shot" was part of the cover letter accompanying the SAS we sent.