Hello,
The new cart system in HikaShop 3 allow to add several products with a single API call ; but there is no new "entry point" in a controller to provide that feature via an URL.
So you still need to use the legacy mode with " &item[20]=1&item[21]=1 " (where 20 and 21 are product id and 1 the quantity ).
And it will be the same for the coupon ; we work hard to improve the improve the core but we first need to keep a good working retro compatibility.
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.