Hi there @fmp,
The reason we publish both monthly & hourly pricing is that a Droplet can exist for only a fraction of a month, so it will need to be billed on an hourly basis.
For each hour a Droplet exists on your account, it will incur the hourly rate, up to 672 hours, where it will then switch to the monthly rate.
If the $5/mo Droplet exists on your account for only three days, you would be charged by the hour ($0.007/hr) and it would cost:
$0.007 * 3 * 24 = $0.504
However, if the Droplet was on your account for a whole month, it would be more than 672 hours, so you would be charged the monthly rate:
31 * 24 = 744;
744 > 672 => $5
From the pricing page:
You will never be billed more than the monthly cost of your Droplet. All Droplets are billed hourly up to a monthly cap of 672 hours (the number of hours in 4 weeks). If you use your server for fewer than 672 hours during the month, you will be billed for each hour that you used it. If you use your server for more than 672 hours that month, you will be billed at the monthly cost. For example, if you spin up a $10/mo Droplet and use it for 336 hours, then you would be charged $5 (going by the hourly rate). If you use that Droplet for 700 hours, then you would be charged $10 (capped at the monthly rate).
Hope that helps!