personally i would not pay in advance, last week we headed off to france, and with the baby we had loads extra, i know that we were well over weight with two bags,, some how we wree able to check in at DUB and no one said anything, apart from they put the HEAVy tags on our luggage, most of the check in is self service now also, so you may just be lucky if its withing reason, BTW the cahrges are in the email they send with your ticket number
Just an update to y own post: I ma in Marbella now. Two out of three cases were overweight (one by 3kg and the other by 7kg) and we didn't charged an extra cent.