The online donations people might recommend this, but in case they don't, the church still supports many donation types via Bill Pay. You have to fill out a form and then there are specific payee names they give you to ensure your donations are credited to the appropriate donation types (tithing, fast offering, etc.).
If changing countries is new to you, make sure you get appropriate financial advice about your tax obligations in your situation. For example, if you owe taxes in Mexico and made charitable contributions in the United States in dollars, make sure that doesn't mess up anything. If you owe taxes in Mexico and they offer any type of deduction for charitable donations, they may not recognize donations in the USA. On the other hand, if you're paying USA taxes, they probably won't recognize donations made to the church in Mexico, and that could be the reason you want to do donations in US dollars.