Because 1 customer would have to generate 49,000 invoices before I start losing money.

Too many businesses are greedy with subscriptions. I won't join the trend.

I reckon the 1600 he's saved for himself already cover most of the costs. You can probably run this on a reasonable VPS very cheaply. Still a yearly fee would probably be more appropriate for the long term I don't think the customers on this are going to be too fussed on the difference between $50 lifetime and $50 yearly given the savings being made