Have you reviewed your PDF invoice in Self-serve? It contains all the fine details of your bill. Your first billing period is pro-rated from when you signed up to the billing date. You are also charged services for the upcoming month in advance This occurs when anyone signs up with a provider. Similarly, when you change a plan, both the old plan and the new are pro-rated for the portion of the month they were active. Here’s link for you describing the process. http://help.koodomobile.com/my-bill/charges-explained
Your first bill is your pro rated amount plus your first full month. Look at page 3 of your bill and it'll break that down. Don't worry, you didn't get overcharged! Your next bill will be the normal amount and all is well.