Currently, if a person renews the *same* product before the first product has expired, it will not add the two durations.
For example, product A expires in 7 days. If I by product A again tomorrow (i.e. a day has gone by), it should update the database by adding the days together, making it 13 days. Instead what it does is give you 7 days again.
So what needs to be done is when smf_store_group_log is being updated after a purchase, look for ID_MEMBER and ID_GROUPTomove columns and instead of adding a new row entry into the table, it adds to the groupexpiredays column the number of days