Is it normal for your customer to demand the terms for payment such as 3 percent net 10 or net 90?

It depends how important the customer thinks he is. Generally speaking seller's dictate their terms and buyers either accept it or go elsewhere. As a vendor naturally I want to keep my customers happy and as a buyer I am also conscious of how my behavior affects my vendor. If you're having difficulty with a customer over invoicing you can play hardball with such sayings as 'if you want the monkey to dance you have to pay the organ grinder' or you can explain a situation that you were involved in where a customer played such hardball that the customer was no longer 'valued' and the vendor actually went out of business. At most I would give 1% 10 net 30, try not to give anything more than net 30, after all you're not a bank, if they want to take a loan they can....