Hello,
The best solution will probably be to use the "Force SSL on Checkout" option via "Hikashop->System->Configuration->Checkout" :
Force SSL on Checkout : This option will force HTTPS on the checkout pages. You need to have HTTPS configured and a valid SSL certificate before you can activate this option, otherwise the checkout will appear as broken and this option will be deactivated by the system. You will have to ask your hosting provider about how to setup HTTPS and obtain a valid certificate for your website. (since 1.0.6)
