I don’t think that this is possible. What you could do is to add account credit to your account so that you have available funds at all times.
Basically, 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 more information you could take a look at the pricing page here:
Hope that this helps!