They will buy the T-bills for you - just tell them how many dollars and maturity. Just email them for details. I'm not sure if they have a minimum acct. balance for this or not. Keep in mind you need to keep enough cash (not in T-bills) in your account to handle any drawdowns in your trading, otherwise they will have to sell some of your T-bills unless you deposit more cash.
It seems to be proportional to the open positions I have. I haven't observed long enough to see the rules. I hope someone else may shed some light on this. The dollar amount could be large if you have large positions.
It always screwed up if you have open positions during their system reset time around midnight. Amount could be huge. I have contacted them for a long time. Somehow it has never been fixed. I don't know why.
