Same here.. I've been wanting to know for ages about all the payment settings spread around in DNN.  I recall reading in teh book that it's legacy and to leave it, but does it work... can it be used.. is it going to stay in there? anyone?

Yes I also hope it's not going to be dropped or anything because I plan to use it myself if I can ever find some documentation or examples how to do it.

The book has only the following in it:

Advanced Settings: Payment Settings
The Payment settings (see Figure 4-17) have been preserved from earlier versions of DotNetNuke for legacy support purposes. Only the PayPal option is supported using the POST method to emulate PayPal's Buy Now button functionality. Currently, these settings come into play only when public roles are defined with fees or when online portal signup is permitted. They are not used in any of the several eCommerce store and/or payment components available through third-party providers or in the free DotNetNuke Store module. These settings may be deprecated in a future version in favor of a more robust eCommerce API.

And in another chapter:

Payment Settings
You learned in Chapter 4 that many of these Payment Settings have been preserved from earlier versions of DotNetNuke for legacy support purposes. For the Host, these settings come into play only as defaults for new portal creation or for portal subscription renewal. These Payment Settings (see Table 5-4) will be deprecated in a future version in favor of more robust eCommerce APIs.

Not the sort of detailed information I was hoping for when I bought it :(

Rob