Account credit is only applied once your invoice is processed at the end of the month.
When creating a new machine, there are pre-authorization checks (which are not actual charges but rather temporary pending transactions that are voided after confirmation, usually within 48 hours). These are required even when account credit is available as a means of fraud protection.
In summary, if you have $10.00 of credit and deploy a $5.00 machine, your card will be pre-authorized for the new machines. A temporary, pending charge will be placed on your credit card that will automatically be voided once cleared. At the end of the month, $5.00 of SkySilk account credit will automatically be deducted from your available SkySilk Account Credit; no charges would be incurred to your personal credit card in this scenario.
I hope that helps clear things up, please let us know if you have any other questions or concerns.