求这段话是什么意思，什么是第三方审核呀，邮件原文请戳标题

到一个客户的邮件，还没始讨论价格，就发了这么一段话过来，我还是个外贸新人，请教各位大神帮我翻译一下，另外我想问一下，这个第三方审核到底是什么意思，是要验厂的意思吗，还什么费用要我们出，求大家给点参考意见。 这是客人的原话 we will need to use a 3rd party agent to audit your factory and the cost will be paid by you, please include this in the cost. the audit fee will be around us$600.00 - this is for reference only. 还有下面这段，我也不是很能理解，求科普啊 open account with 30 days to start ( we will discuss how many days later when we start business .??it means after shipment you will send full set of original shipping document to our office for checking, after approval , we will count 30 days then make payment. 客人说这是他们公司的规定 是表示只接受的意思吗，还是说到货后，30天内支付呢？