Hi, this is my first month using digital ocean. So, I do not quite understand the payment system here.

I am using the $10/month.
I got a promo code $5, so I just sent $5 from my paypal.
And now I get an invoice $6.22 for 01-14 13:34 to 01-31 23:59
For February, I’ve already got $3.21 and increasing time by time.
My balance $0.57 now

Can you please explain how do you calculate the payment?
How to make it fix price $10/month?

Thanks a lot