Is it safe to accept money in the mail?
The US Postal Service to my recollection recommends only mailing cash or items with cash-like characteristics using Registered Mail service. Registered mail is expensive and a pain in the butt for everyone, as it requires an audit trail for each individual who touches the mailing.  If you're doing a lot of business and word gets out that you're accepting cash payments via the mail, you'll probably attract unwanted attention from the tax authorities as well. It's fairly unusual.