Rubbish. In these things there is always plenty of leaway for the mail to catch up. Pay on time and send in application ASAP. I have paid and yet not mailed application but even with snail mail as we have it today it will get there before they need it for the concentrator.
These forms for application come from the dark ages before computers and direct crediting so still say things like must be in by such and such a date. In effect legally only the money has to be in on time and if the snail mail takes forever and a day no company would turn down an application if the money is on time.