Santu allows you to approve manual payments yourself - or to use a third party payment service, such as PayPal, Authorize.Net or WorldPay.
If your customer is having problems getting for example their credit card details approved, then this will most likely be, because the service provider you are using does not approve the payment.
This could be because the customer is incorrectly providing the card details, because there are not sufficient funds available or because the customer's bank does not allow online transactions above a certain value or from othe countries.
In all cases the customer should contact the bank and inquire, why the payment is not being approved.
Santu itself does not approve payments.