You will need to Set Group Limits so that users have a monthly or one-time balance for sending gifts/items. This balance is displayed to each user and used for gifts, postage, etc. When you set a Monthly Budget Limit (i.e. $1000) then each user in that Group will have a max budget of $1000 per month. You can also decide if that balance resets or rolls over each month.
At any time you can also update a users balance and give or take away funds from their account: