Amazon has a gazillion customers.
They use email for usernames and any simple password you want.
It works just fine.
Don’t let your techies convince you that you need some crazy-complicated password scheme. You’ll just annoy your users and make it less likely that they will come back.
(Unless you have something extra-super-sensitive. But if you’re just selling stuff, you don’t.)
If I can’t remember my password on your site, I’m going to buy from Amazon instead.