so when your send your ACH payment to them, they don't actually have the funds yet? how do you know this? this is an issue for the client then.. why would i want to pay for something that someone doesn't even have? that's like asking me to prepay for a service that i haven't even yet received.
An ACH transaction takes 3-5 days. They don't give you the bitcoins immediately because the ACH transaction can be cancelled during that time. Furthermore, an ACH transaction can be reversed. They must be cautious. Otherwise, scammers will use the banking system to steal money from them and put them out of business.