this is how i would put it.....

Based on the fact that you have been asked to use "predocumentación" rather than "despacho previo", I have a feeling that "advance documentation" would be the best term for you, since it is quite general and can cover both the bill of lading and other documents submitted in advance of the shipment. Actually, I think that the expression "despacho previo" corresponds to "advance manifest", and the "despacho" is the cargo manifest (the document that lists what the carrier has loaded for import, into Mexico, in this case). Indeed, the Union Pacific site that refers to "despacho previo" as the "southbound process", also has a section on the Automated Manifest System (AMS) for rail import into the US (from Mexico or Canada). The Spanish version of the site calls this the Sistema de Despacho Aduanal Automatizado:
http://www.uprr.com/customers/mexico/ams.shtml
http://www.uprr.com/customers/mexico/espanol/ams.shtml

So I think "advance manifest (system)" would be quite accurate for "despacho previo". However, the more general term "predocumentación", as I say, would be better rendered as "advance documentation".

Here's a World Shipping Council document on advance manifests, which contains the following comment:

"For example, a European exporter shipping its goods to Latin America may refuse to use a service that requires such advance documentation if it can use a service that doesn’t call in the U.S. and thus doesn’t require early cargo documentation."
http://www.worldshipping.org/pdf/customs_manifest_comments.p...

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Note added at 7 hrs (2012-05-25 10:53:07 GMT)
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To develop my response to John's disagreement, let me make it clear that I am suggesting you use "advance documentation" as the translation of "predocumentación". As I have already said, what we are talking about here is in fact an advance manifest system. The US has what they call the AMS, which stands for Automated Manifest System, for incoming rail freight. "Advance manifest" is a generic term used in relation to other countries, such as Turkey, China and the European Union, as well as the US (and Canada):
http://www.apl.com/advisory/html/advisory_20090630.html
http://www.oocl.com/eng/resourcecenter/customeradvisories/Pa...

So actually there is no reason at all why the term should not be applied to Mexico. There would be no danger of confusion, since the whole context will make it clear that it is referring to imports into Mexico, and an advanced manifest is simply a cargo manifest submitted in advance of import.

But in any case I am not recommending "advance manifest" as the translation here, for the reasons I have already stated, since the documentation includes more than just the manifest.

It is presumably for this reason that your client doesn't want you to use (or translate) "Despacho Previo", but "predocumentación". The system, as John says, is widely known as "Despacho Previo" in English-language US texts, but your client doesn't want you to use that. What you need, then, is a brief term that corresponds to "predocumentación", not an extended explanation. I think "advance documentation" is the obvious choice.

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Note added at 7 hrs (2012-05-25 10:55:57 GMT)
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My reference to the US AMS was included simply to indicate that "despacho", in "despacho previo", really means "manifest".

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Note added at 7 hrs (2012-05-25 10:59:00 GMT)
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Depending on the particular contexts in which the term appears in the contract, you could used "advance documentation system" or "advance documentation process". You need a noun, and you're not going to want to mention "Mexican Customs" each time; there's no need anyway.

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Note added at 9 hrs (2012-05-25 13:22:53 GMT)
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Further examples of "advance documentation" in relation to imports. This is from another World Shipping Council document:

"For compliance with requirements regarding the advanced electronic filing of inbound cargo information, CBP has established a comprehensive set or regulations that require advance documentation of all containerized import cargo regardless of commodity or shipper."
http://www.worldshipping.org/pdf/trade_act_regulation_commen...

One from the OECD:
"Such systems also do away with unnecessary burdens on traders by downscaling physical inspection and reducing bottlenecks at border crossings. This is one of the most costly trade facilitation measures as it requires investment in infrastructure and specialist training. But it also produces significant benefits, not just in reducing the number of cargoes that need to be inspected, but also in speeding up the processing of advance documentation."
http://www.oecd.org/dataoecd/58/25/35459690.pdf