Costco has iPhone 11 64GB on Sprint network for $0.00 per month when you add a new line to your existing account or open a new account.
6s or above Trade In required.
This is not Sprint Flex Lease, it is the monthly installment plan for 24 months with a bill credit of $29.17 per month making the iPhone 11 64GB free.
The cheapest plan I saw was for $65 but that might be YMMV based on your account status
Plus a $100 Costco shop card