Jaime,  Attached is a firm sales agreement form which ECT utilizes in  North  America.
A lot of the form may not be relevant given the fact that (i) Pemex  is both the seller and transporter and (ii)  unless you are near the border,  there are no alternative sources to Pemex.
However, it is a good starting  point.
Let me know if you have any questions.

Firm Sales Form