If you go into the online payments section in 3dCart ADMIN, you can check or uncheck "Authorize Only" That way you can keep from capturing the sale. This is very important too for Visa/MC rules that you must ship within a certain time ( I don't recall the exact time now - I'm thinking it is 72 hours of capturing a sale) You can also go into your Authorize.net aacount and void the transaction. I know with the Authorize.net account we have you can put it into test mode.
I hope this helps some. Good luck with the site. 3dCart is really a cool cart to work with.
Bill Fox
http://www.4filters.com