Replenishment's work exactly the same as new activation's, funds need to be available on your prepaid balance.
For customers setup on auto pay
If the customers card is approved and the funds are not on your TK balance then the replenishment will fail. Should the replenishment fail, the customers charge will NOT be captured, only authorized, in which case it would fall off of their statement 2-7 days later automatically.
You can also replenish an account without setting up your customer for auto pay by simply going to the order and clicking the "Replenish" option from the "Order Options" drop down. Again, funds need to be available on your TeleKloud balance.