Personally I would negotiate my hourly fee based on the project extension in time, e.g. a six months consulting contract has a lower hour rate that a few days contract. I tend to not negotiate the payment terms though, which are always 7 days following the receipt of my invoice.
In Italy very often delayed payments are an issue, but for my type of consulting the customer knows that's better not to delay my payments due to the confidentiality of the matters which I deal with.