Hi,
The goal of Akeeba Subscriptions is to store the subscription of each user and know the different dates.
So I will recommend you yo use a subscription component for this mailing purpose. Akeeba is for the moment and for what I know, the only right integrated component with HikaShop.
If Akeeba Subscriptions does not have a feature to send an email X days before the renewal, I think that you can do it using Acymailing and the "auto newsletter" system.
This Acymailing feature is very powerful and allow you to send automatic email depending variables, you can plug it to HikaShop or a lot of other extensions.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.