Sending Money: Xoom is a good option to send money. You can do it online, and send it directly from your back account to a pickup point in San Blas. (Coppel and Azteca are 2 of the options). You can also send it to your Banamex account.
Papers: You need a tourist visa to visit San Blas. You can get it at the border or at the airport or on the plane for international flights. It costs around 20 USD and if you ask, you can get it for up to 6 months. They will not ask you for it when you enter Mexico because for most the areas around the border you do not need the tourist card. You must ask them where to get it and how to pay for it. If you forget, or your visa expires, you can go to Tepic or Puerta Vallarta to get one, but this is a huge hassle. If you want to live down here you will need to apply for a termporary or permanent visa and show proof of an income. For the permanent visa if you are not already living in Mexico you must apply at a Mexican counsulate outside of Mexico.
Passports: You need a valid US Passpor to return to the United States. If you lose your passport, or need a new one there is a US counsulate office in Puertto Vallarta.
Cars: You can bring car with you on a tourist card, but you will have to register it, and leave a 500USD deposit, either in cash or with your credit card. If you do not return the car on time you will forfit the $500 and not be allowed to bring another car into Mexico. You should get Mexican car insurance. If you are a permanent resident you cannot drive a car with US plates. At this time it is not possible to "import" a care with US plate and get Mexicaan plates for it, so you just buy the car in Mexico. I think there might be one exception, if the car is exactly 10 years old.
Dogs: You can bring you dog to Mexico. Always carry proof of vacinations. Dogs are welsome in many restaurants. In San Blas, the Hotel Bucanero accepts pets. We found that traveling outisde tourist areas in Mexico it can be quite difficult to find hotels that accept pets. If you look at booking.com they have a search feature that shows what hotels accept pats.