Thanks Justysia, I have just checked on the Easyjet website and found this info...
.....you have to go to the airport sales desk first and pay for hold baggage not already paid for previously. The fees are higher at the airport. easyJet recommends to pay for hold baggage online or via the call center at the time of booking......
.... so I think that they were just pointing out that fees are higher at the airport so it's best to pay when booking. I won't be charged a second time. :-)