This is the pro feature of the plugin.
After login, users of your store will navigate to his “Funds” from menu in his “Account” page.
There he will get a field like below, where he will have to put the desired amount in the field that he wants to topup.
Clicking the topup will redirect him to the cart.
From there he proceed to checkout and order.
After admin completing the order, the topup for that user will be completed and he will have that amount in his account for future use.