The link provided say to wait 4 hours to schedule interview after bank payment, So I don't see it being an issue, My wife applied thru the www.usgov.org link which didn't say to pay first, like the above link does.
We are not in a hurry for her mother to come here, We want her here in Texas in April of next year, it will be cooler then and let her stay a couple of months and then we will fly back with her to Philippines in June and me and wife will spend some time exploring Philippines. It's too hot now in Texas and we have travel booked in October to Cancun and December to Hawaii and then winter sets in and not a good time to be a tourist in Texas.
Plus my wife pinay friend mom is coming in April and we are hoping they can fly together from Manila to Korea to Dallas, and no her mom hasn't applied for tourist visa before, my wife has been in USA going on 10 years now.