Although Viaweb supports real-time credit card authorizations through Cybercash, we do not recommend it for most merchants.
Few online merchants actually need real-time credit card processing. Most of our users, including nearly all the larger ones, don't process credit cards in real time. They don't need to. It works fine to simply to get batches of orders, including credit card numbers, and process them using conventional technology.
The one exception is merchants selling downloadable products. Software, for example. They need to process the transaction in real time because they are going to deliver the product in real time.
For other merchants, using Cybercash tends to be more trouble than it is worth.