When purchasing (or upgrading) an add-on package, you'll be charged with a prorated fee and granted with a prorated quota, according to the remainder of the current billing cycle. Note that this is stated as well on your purchase page, just before you approve the payment.
If you lack quota for the current month, you can upgrade to a larger package. Your quota then will be granted with credits under the same policy as above.
Note that while upgrading takes effect immediately, downgrading will only reflect on the next billing month.