Typed this as Roseway was posting, but I'll post it anyway!
As long as your wireless network is encrypted with a STRONG WPA key, basically the more characters you use in the key the harder it is to crack as so many more combinations are required to brute-force an attack. WEP is pretty useless for any form of security as it can be cracked very quickly nowadays, even 128-bit WEP.
Most routers also allow you to look at the list of currently connected wireless clients, so if your router lets you do this, and you want another check, you can make sure you don't have another machine connected before doing the transaction.
And of course all credit card transactions online are (or should be!) done using SSL which means your credit card number and personal details are encrypted anyway. So even if someone actually manages to get onto your wireless network, they've then got to decrypt the secure HTTP session as well - I think they'd have to be quite determined to target such an attack on a single residential wireless connection!
It's highly unlikely that, even if someone does get into your wireless network, that they'll be listening at the particular moment you send your card details over the net. I guess it's more rewarding for someone to gain access to a whole list of credit cards from a particular company's database, which seems to happen more than I'd like, even though it isn't reported too often.
So yeah, use WPA together with a long wireless key, that also contains a few symbols, numbers and spaces (or better still, WPA2 if your router and wireless clients support it, not all do), and you should be pretty secure.
Hope that helps
