Mailed $20 to b2c earlier last week. Hopefully it will go thru soon. I'll follow up here when the
transaction is complete.
My letter to bitcoin2cash wound up returned just like jimbob's

It was addressed exactly the same way as his letter.
My guess is that TTBit is right that USPS might get suspicious when the business name doesn't match the address. Leaving out the name entirely is probably the right way to make our letters containing cash as inconspicuous as possible.