You can search back across this board and see this discussed at length many times. There are members of the church who believe in their heart of hearts that a recurring payment DOES NOT EQUAL a donation made in faith... STRONG feelings exist that an act of charity, an act of faith can't be scheduled every 2 weeks as you receive a paycheck.....
I find that sort of thinking rather strange, especially if your income each month can provide the funds needed to keep a missionary in field, or if you and/or your missionary have saved the needed funds to serve a mission in advance.
However, when I was young and before each mission cost the same, we had three missionaries serving from our family at the same time in very expensive missions. The total cost per month for the three was almost as much as my father's entire salary! Countless people chipped in and helped, the cows gave extra milk, our clothing store made a higher profit, etc. To this day one of my aunts has a notebook listing each missionary in our family and how much she donated to them each month. Without her help, and the help of countless other relatives and ward members, we simply couldn't have afforded it.
My parents started each month not knowing how they were going to make the payments for three missionaries, yet by the end of the month the money to make those payments was always there. Many, many people were blessed (and blessed us and the missionaries in our family) with their constant generosity. Those were certainly acts of faith and charity, and there's no way that my parents could have set up an automatic payment plan with no idea where the funds for it would come from each month.
However, I diligently saved all the funds for my daughter's mission before
she received her mission call. So theoretically I've already made a huge
act of charity by designating those funds to pay for her mission. I don't have to worry where the money for her mission will come from each month, and an automatic payment plan would be a relief.
Were automatic monthly payments for missionaries allowed, the email I'd get each month from the Church that says they've received and processed my donation would remind me of the ongoing charitable act I'm performing throughout her mission. Since I have to remember to log on to LDS.org and make each monthly payment in person, that email just makes me sigh with relief that the church has received her monthly payment. (Which in my opinion isn't a spiritual experience, but your results may vary...)
I'm very PROUD that my daughter is serving a mission, and I'm grateful that I have the means and the privilege of paying for her missionary service. I receive numerous blessings as a result, the greatest of which is a daughter whose testimony is growing as she learns many of life's tough lessons, and she'll be an even more wonderful person when she returns.
However, I doubt the blessings she and I receive thanks to her missionary service are affected in any way by the method in which her monthly dues are paid on the LDS.org website. But that's just me...