These refill scripts are also needed at the border if they ask for them. Purchases above that will be taxed at the border. Most shops take American money but often don't have change for large bills. I find that using ones, fives and tens I can usually do alright. A twenty often causes problems in the smaller shops who do not keep large amounts of American money on hand.
Some shops also take checks or charge cards.
If you are traveling farther South than the free trade zone you must make arrangements for visas, automobile permits, insurance, etc.