It seems you could sell the mailbox device and the gateway together if cost permitted (I know you say "No base station required" but some users may not mind running a basestation, especially if it required little/no configuration).
That would cost a lot of money and have no content. The problem isn't friction. The friction would be easy to overcome if you didn't have those two fatal problems.
There are gateways who are more complicated and more for developers, but of course i don't want to offer these.