Hi Stephanie,
Great question. For Amazon orders, most people use a service like Jetbox.com, which has offices throughout Costa Rica. They’re kind of like a private courier service. Your Amazon order is shipped to their place in Florida and then down to their office in Costa Rica. They take care of all the customs as well. They would be best equipped to help you figure what the customs fees might be.
I haven’t heard of any particular problems ordering vitamins through Amazon. But keep in mind that Costa Rica has a big tradition of natural cures and such. There are stores called “macrobioticas” that are basically natural pharmacies. They carry herbs, vitamins, oils, etc. So you might be able to find what you need in-country.
In most stores you can find lentils, kidney beans, and garbanzos. You have to go to speciality stores to find tofu and different varieties of lentils. I haven’t yet seen tempeh, unfortunately. That’s one thing my family misses from the U.S. as well. It’s possible there is a specialty store I haven’t found yet that carries it but so far no luck.