Hi,
We are currently working on redoing the whole add to cart and checkout system. It's a really big development and thanks a lot of time. We started working on that more than 6 months ago and still have a few weeks/months ahead to finish it.
The new system will allow the add to cart URL to support coupons and products at the same time among a lot of other things.
It seems that Hika isn't either.
Thank you for your kind comment...
We also sell addons for HikaShop, like HikaMarket or HikaSerial (and more are to come). This has nothing to do with enabling the adding of coupons through the add to cart URL. Please stay on topic. It's not because one feature you need has not been added yet that you should talk like that. If you really need it, you can just have it developed. If you have the money for it, there will be many happy to add that feature for you.