I have made two trips and was advised that you may have to pay duties on any large amounts of items. The customs form stated a person was allowed 12 rolls of photographic film. I don't know how much this would be for sheets. If I were to stay a month I would use a lot more than 12 rolls!
Maybe you can purchase everything in Mexico.
I too got the info that only 12 rolls of film were permitted. I carried in about 20 rolls, for only a week's stay, and no one at the airport even so much as glanced at it. If it were me, I'd chance carrying it in; if they charge you duty, it will at least be done on the spot. If you ship it, and it lands on some bureaucrate's desk, your month will be over before you get your film. On the other hand, not having shipped anything to Mexico, what do I know?
David, don't, Do Not ship any photographic material throughout customs in Mexico, the bureaucracy is terrible, you gonna spend a lot of time trying to take your things out.
are you driving into Mexico or are you flying in? the reason I ask is that driving in, my truck was looked at only once and that was because he wanted to see a box that I had my 8x10 camera in. I had film all over the interior of the truck and no one said anything at all. When I last flew in to Mexico they made me fill out some forms and they just looked at it and checked it off. But I did not have 200 rolls in my bag either.
You won't have a problem carrying the film in I never do and I go there frequently. They don't even question it, and frankly you don't need to mention it. As far as the chemicals, possibly the closest big city to where you are staying can provide those. Mexico is a lot more Americanized than people realize. There's Home Depot, Sears, Penny's in Cancun.