Sum credit and debit transactions where credits have expiries

Question

I'm building a limiting system that we can use to control how many emails a user can send from our platform. They automatically get granted 10,000 emails per month on their billing date. And they have the option of purchasing more in 5,000 email packs. My idea for this was to add the limit credit and debits to a table, and then use SUM(amount) to calculate what their usage is at any given point in time.

This is complicated by a few requirements:

1. The user automatically gets 10,000 emails per month. When we calculate the user's limit as it stands right now, it has to look from their last billing date onwards.
2. The addons that a user can purchase to increase their limit have an expiry date, if they have not used the full amount credited by the addon by expiry, they lose the remaining amount.
3. Addons should persist forever until they are used up, but the 10,000 credits per month reset every month.

Currently, I have this table structure made for a proof of concept (info column added just to explain what's going on):

And this query to calculate the user's current limit at the time of running the query:

SELECT SUM(amount) FROM (
    SELECT SUM(credits.amount) as amount
    FROM limit_transactions credits
    WHERE date >= (
        SELECT date FROM limit_transactions WHERE limit_transactions.billing_reset=1 ORDER BY limit_transactions.date DESC -- to start from where the billing cycle reset
    )
    AND credits.amount > 0 -- to only get credits
    AND (credits.expiry IS NULL OR credits.expiry <= NOW()) -- to exclude expired credits

    UNION ALL

    SELECT SUM(debits.amount) as amount
    FROM limit_transactions debits
    WHERE date >= (
        SELECT date FROM limit_transactions WHERE limit_transactions.billing_reset=1 ORDER BY limit_transactions.date DESC -- to start from where the billing cycle reset
    )
    AND debits.amount < 0 -- to only get debits
) as amount

I think this query is going in the right direction, but is giving an incorrect result. With the given data set it returns 2000, when it should return -3000. This is because this part of the query:

AND (credits.expiry IS NULL OR credits.expiry <= NOW()) -- to exclude expired credits

is completely eliminating the addons that have expired from the overall calculation, instead of being aware of what debits have occured while that addon was unexpired.

Has anybody implemented a solution like this before - or can suggest a better way to model this data structure? The other way I could go is only storing credits in this table, and adding a used column that would track how much of that credit has been used. If a user sends an email, I would increment the used column.