My experience is that once a credit card of mine was declined on the EK website, it was then 'blacklisted' and I could never ever get it to go through on the website again.

However, to avoid the 5% fee, you can still process through the website the ticket you want. If it rejects, the system still gives you a booking reference, which is good for 24 hours. You can call and ask to pay that reference over the phone and tell them your card was rejected and there should be no fee. I had to do this for one year until I got a new credit card and the system accepted me again.