Clients and multiple accounts

I'm building a database for my personal training business, and currently I
ran into a huge pickle and don't know how to bite it. My problem is this, a
client purchases a certain number of sessions that expire after a certain
amount of time. I figure I need an account that records the number of
sessions paid for and the date that they expire. After the sessions expires
the remaining sessions are then moved to a savings account kind of, that is
refunded only after accumulating to a certain number. The price of each
session is variable, depending on the package the client signs up with, so I
need a method to be able to turn session numbers back into monetary value.

So far I decided to create an invoice table that records every paid order;
along with payment date and start date, and then with a query I calculate the
end date. For example: Payment starts on the 3rd of the month, client pays
for 2 sessions a week and they paid for 4 weeks of training; so they receive
8 training sessions that expire on 31st. My main issue is setting the
database up so every client has a session account (including reserved and
expired sessions), that increases every time an invoice is paid, and expires
so that their paid unattended sessions are moved to another field (expired
sessions). I also want to be able to still keep the market value of each
session (reserved or expired), because some clients might pay for sessions
but never use them. I'd like to be able to refund the money.

The query does all the calculations but I'm stuck as far as moving the
information into an account table or such.