A fee is charged by all of the online payment systems to check that you've given them a valid credit or debit card, or bank account for direct debit, normally (but not with every service - e.g. pay-up-pal) it is credited to your online account.
So if they do a test transaction of say 49p, you use that value as a confirmation that it is your card or bank account and they then put in into your account as if a buyer had bought something.
As long as you then have other deposits and can build up a minimum transfer balance, you don't lose anything.
:!:
:idea: I extracted the current costs and fees etc from the major service providers and put them in a table on my website at -
http://www.gazlannathai.com/Auctions/payonline.htm
so that you can check them at a glance.
It's not a complete list as the site is still newish - but watch it for updates.
Regards
Gaz