DO reserves say $20 (just an example) when one spins up a new droplet, and if one forgets to pay — then, DO shuts down the server. But keeps the backups, until the GB-month cost is $20.
One could choose how much money to reserve, depending on how important the data one stored on the droplet, was. If it's just for running test: $0. Customer data: Maybe $$$ instead.
There could be a default that made the data and backup last for 2 or 3 months?
And any remaining money would be refunded, if one closes one's account or sth like that.
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Actually I'll look into implementing this, in my own SaaS (which, like DO, takes monthly recurring payments and stores customer data).
I believe a solution like this is an elegant way to ensure that the customer gets what they 'believe' they are buying which is a sure-fire way to keep the customer happy.