求翻译：我恐怕我们无法提供形式发票号码，因为发给客户的货品是根据先入先出的原则发货的，所以客户投诉的产品往往无法追溯到是具体哪一单货。是什么意思？

I am afraid we can not provide pro forma invoice number sent to the customer's goods are shipped in accordance with the principle of first in first out, so the products are often of customer complaints can not be traced back to a single shipment of concrete which

I am afraid that we are unable to provide pro-forma invoice issued to the customer number, because the goods are on a first in, first out basis for the shipment, the client's products are often complaints can be traced back to a specific, single.

Perhaps my we are unable to provide the pro forma invoice number, because issues the customer the goods is according to the principle which enters first leaves first delivers goods, therefore which single goods the customer suit product often is unable to trace is concrete.

I am afraid we are unable to provide the invoice number, because the goods are given to the customer according to the first in first out principle of shipping, customer complaint cannot be traced back to specific which is often a single goods.

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