If you live in Mexico city why are you going to Jaurez? Sounds like a waste of gas money, or you're bullshitting. Your already in Mexico, no need to go to the border.
Dude you've obviously never been to Mexico City and don't know too much geography. Mexico is not Puerto Rico, no one drives from Mexico City to Juarez. I have to go to Juarez anyway for another matter, so might as well do some comparison shopping while I'm there.
Since Mexicans are not big roid users you don't have anywhere near the selection readily available here that you do on the border where every gringo goes to do their shopping.
I can order Animal Power, Quality Vet, and Denkall (directly from the manufacturers' rep) easily here in Mexico City but want to compare to competing products' prices in Juarez. Also, these manufacturers do not have every product I would like to buy.