When you start receiving stuff should depend on your fictitious baby's due date. It wouldn't make sense for a company to send you baby formula or coupons long before the baby is due.
I get the most stuff (formula and paper mail) from the main company. Once in a while, I also get paper mail (including coupons) from other baby-related companies.
Yes, it is often a pain to mail things to the MSC. I often check my boxes after business hours. It's not a problem most of the time since there's nothing to mail most of the time. When I do find a slip in the box from the post office, however, that means I have to go back another day during business hours to pick up the package. I have to make a third trip sometimes if there is a long line at the post office on my second trip and I can't wait.
I love the $20/month for each box and don't feel guilty at all when I receive no mail for months for any one box. Knowing a shopper did not get any mail is also valuable data to the MSC. The important thing is we check when we are supposed to and forward any mail and data to the MSC.