If you're moving freight into Mexico, the single most common reason a cleared load still gets stuck is the Carta Porte — and most US shippers have never heard of it until it bites them.
The Carta Porte (formally the Complemento Carta Porte, attached to the CFDI electronic invoice) is a document Mexico's tax authority, the SAT, requires for goods moving on Mexican roads, rail, water, or air. It proves the legal transport of the cargo: who's moving it, what it is, where it came from, where it's going, the route, and the vehicle and operator. It's been mandatory and actively enforced through its current version, and the penalties are real — a load moving without a valid Carta Porte can be detained, and the carrier (not the shipper) can be fined or have the goods seized.
For a US shipper that's the trap: you handle the US-side paperwork cleanly, the load clears customs at Laredo, and then it stops 40 miles into Mexico because the Carta Porte data didn't match the physical load or was never generated correctly. The cargo description, the SAT product code (clave de producto), the weight, the origin and destination, and the operator's RFC all have to be exact.
This is why a real cross-border desk generates the Carta Porte through a named Mexican customs broker (agente aduanal) before the truck reaches the bridge — not after. We coordinate the data so the commercial invoice, the USMCA documentation on the US side, and the Carta Porte on the Mexican side all describe the same load the same way. When they match, the load moves at inspection speed. When they don't, it's the most expensive idle hour in trucking.
The short version: if your broker can't tell you who generates your Carta Porte and how they verify it before dispatch, that's the flag. Cross-border isn't one operation — it's a US trucking operation and a Mexican customs operation stacked, and the Carta Porte is where the Mexican side either works or doesn't.