Most agents should allow you to withdraw your money back to your account from the balance page (Superbuy, Wegobuy and Pandabuy certainly do).
This can be useful if you returned an item, got a shipping refund or just have some leftovers.
Usually it has to go back the same way, meaning if you topped up with paypal the withdrawal will be issued as a Paypal refund.
Don't expect to get back 100%, one way or another the shopping agent will take their cut. Generally you should try to transfer the agent as little money as possible in the first place, instead of trying to return it late.