Hello,
If I use the ims_after_add_to_cart I have to recalculate everything, and resave my data to the database. Most of the code will be nearly all copy and paste what you have, and therefore just being re-executed.
Would you please reconsider adding the hook ims_after_update_cart in the location I suggest as this would be a massive help in the creation of the CD plugin, as with this addition it is complete. (I have added it myself and it all works).
Thanks,
Leon