'At your command' is correct and can be used in written English.
This phrase is typically used to show deference to someone in a position of authority. For example, "At your command, I will do whatever is necessary to complete the task."
At your command, an electronic agent will scurry across the Internet to find and deliver what you want.
"I went only at your command!
Our instructors are at your command!
"Can you make a thirty-cubit putt drop at your command?
You have to use patronage, personal persuasion, threats, whatever is at your command.
The town design is also at your command, with the help of lovely Isabelle.
Those in attendance said yesterday that each and every livery cab was a potential target, a stop-at-your-command cash box in which the customer faced the back of the cashier's head.
At your vocal command, a friendly, recorded voice reads you the latest information (when you say "Travel," "Traffic," "News," "Stocks" and so on) or location-specific information ("Movies," "Restaurants," "Driving directions").
This translates to "at your command/at the command" meaning that if there's anything else you can assist with, you will be very much willing to do it at the person's command.
Subject: Naval Constructor H. A. Evans, U. S. N., detailed at Norfolk Yard. 1. Naval Constructor Holden A. Evans, U. S. N., has been detailed at the Yard under your command, to assist you in carrying out the Department's written instructions in connection with preliminary work of introducing modern management.
The new Siemens Gigaset 4215, for example, is simply a cordless phone that can dial up to 20 people at your voice command.
