Only Microsoft knows when the next promotion will be. In the mean time might I suggest you try the Xbox Rewards website to earn points by completing certain tasks like filling out surveys or playing new games.
MSP have been retired and you should now have local currency in your account that was converted from whatever MSP you had. If you have another payment option on file, you can go through the process of purchasing the contant with that and Xbox should pop
up a window asking you if you would like to use the balance of your account before your other payment method is used. I know this option is available for the console, but I don't know if they have this set up on the website.
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