[siong1987]

You don't have to worry about scaling it to hundreds of millions revenue a year if you are not running a startup.

In this case, it seems like the OP isn't running Sendy as a startup. He is working on Sendy as a lifestyle business which is making more than enough money for the OP and maybe more than some of us who are working on our own startups.

[ujal]

I disagree here. If growth is the main factor for a startup, charging one-off fees guarantees you not to be delusional about your growth. It is actually much easier to get stuck with the recurring fee model, where you reach ramen profitability but don't grow as fast or worth - not at all.

EDIT: Actually the same psychological effects that push entrepreneurs to charge a recurring fee, push the customers to prefer a one-off fee.