Thanks to your answer, I found the following link which provides more information about how lightning invoices are generated: docs.lightning.engineering/the-lightning-network/… and github.com/lightning/bolts/blob/master/11-payment-encoding.m‌​d

@ng.newbie While a client is free to set any fee they like, miners (who ultimately include your transaction in the blocks they mine) are also free to reject it. Each block has space for a limited number of transactions. Transactions with higher fees get priority. Transactions with no fees usually get ignored.