My husband USC doesn't get pay stubs/ cheques, he pays taxes and we have the passed 3 returns. Will the letter from employer stating info and his average weekly wage be sufficient?

It should be. My father was my co-sponsor, he has his own practice, so he just had his partner write a letter... so far no complaints...

I thought that as long as he has the letter from employer explaining everything then that should be enough to satisfy them, but who knows, they're a fickle lot - INS

This should be sufficient, but the company must have copies of checks, even if it's something they keep in the computer and can print. They must keep a photocopy or some kind of record. For your peace of mind I'd call the payroll department and request copies. I just wrote a similar letter for a coworker's wife's AOS, and I think a well-written, detailed letter will do. Good luck.