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It doesn't matter if the user is online or not ... the user should be able to set a specific amount of stamina they want to use on buffing other accounts.
If you have a package that costs you 100 stamina .... then you should be able to say "Hey, I only want to sell 10 of these packages."
100 x 10 = 1,000 stamina
The player should be required to have the stamina available at the time of activating the package.