Sure, it's legally possible. Do you want them to have a stack of cash for you when you're done? Then make that be a requirement in the contract. I'm not sure how you would enforce it - charging a penalty for late payment makes the most sense.
But most companies don't have that much cash on hand without preparation, and keeping a lot of money around for all of the vendor payments sounds very risky. Most people would feel safer paying by check or wire transfer. Of course, that might take a few days if you get paid on a bank holiday
What if the person you've been working with is sick that day - who can confirm that the payment is authorized for that day? What if your invoice arrives at the end of the day just before everyone takes a week off for Christmas? Or on the day of the company picnic, or there was a blackout.
Basically I'm saying that expecting to be paid on the same day is organizationally difficult for the general case, and therefore unreasonable. Net-15 is reasonable, although if there's only one person handling accounts then it becomes a bit trickier taking two weeks off for holiday. Net-30 has been standard in my experience and it's what I charge. I've had one client do net-60. That was a right nuisance.