Shipping li-ion cells?

what shipping service should I use to ship 50 li-ion cells? I remember when I tried before usps having a problem when I told them what it was and they sent me to fedex I think it was

I’ve received plenty of LiPos through USPS and I was just looking this up a week or two ago and I believe you just pay a slight premium, like $10 instead of $5 for ground service because they can’t ship them via air.